Report No. AOOO 00 0291 •
Department of Water Affairs Department of Water Affairs Republic of Botswana Republic of South Africa
Joint Permanent Technical Committee on Water Affairs
JOINT UPPER LIMPOPO BASIN STUDY
r------·- ______-,Stage I ,., _.-. . l
! '.. ~ I." .'--- I v_: ,
WATER RESOURCES
1991
MacDonald Shand Consortium
Sir M MacDonald & Partners Ltd Ninham Shand Inc.
PO Box 2466 PO Box 95262 Gaborone Waterldoof,0145 ]111 Botswana. LA}! South Africa. A.,... "' ·F.x Cl WATER RESOURCF..s
P:lIC No EXECUTIVE SUI>L\1ARY
ABBREVIATIONS
CHAPTER 01; !N1'RODUcnON
01.1 G·'
CHAPTER 02; BOTSWANA GROIJM)WATER mmy
G2.1 Inooductia1 G·' 02.2 Hydros;colo,bl Etlvironmenl G·' 02.3 S"niflCltlt Aquifers G4 02.3.1 W:IIcrbcrJ FOlm:uio.... G4 02.3.2 Tr.In$V.1.1l Formations G4 02.3.3 Karoo Form:llioos G4 02.3.4 Sand River ~ Alluvium G·' G2.4 Ground..,:II., R~ es: Current Snu:uiOll G·' G2.U Ocncr.ll G·' G2.4.2 TJ:IIIS.:s:>i Form3lions· Lob'lItc Arcl G< G2-4.3 Tr:IIISY:II:lI Form:IIionl' R~ ..... Arc3 G·' G2-U TI3I\SY3:II Form31ions· K3nye AIel G< CH.S Wataber, Form.llion . Mcchlldi and Mokpolole G·' G2.4.6 W3teI'bcrJ Form.llior\s • P:113p)'1l Are:! G·' G2.4.1 K""lO Form:llioo . Mm:uoobtlla A:e:I G·' G2 .•.8 Sand Riven . Lo"'Cf Stwlle. MolloulSe and Mah:Ilaps,"c G·IO G2.4.9 AlIuvi:ol . T'lbn3 Md Dil:lblOnll Areas G·]O G2.~ Grouoo..,ater Resources: Future Pomnti:ol G·] I Cl.S.1 Oc ...13l 0· ] I G2.S.l Karoo Ftm>:lOOns Ww of Scrowc. G·l] 02.S.l Karoo FtmuIions in die: LelJh.lkcnl· Belllh3p.nlou G·]2 02-S.4 Karoo FQrrn3Ilons • Mm3lOObu!.1 Att<1 G·]2
02~~ K3IQO F , LIST OF COI'lTt."ITS (CONTINUED) CHAPTER Gl: SOU'T1i AFRICA..'" GROl,NDWATER Sl1JDY G3. I Introduction G·u 03.2 H.>nl·roek F<:ml=O:!s G·16 G3 .2.1 Crocodik-M:Irio:o 1o..,1:wL G·16 G3.2.2 M:ltbbou C.:ltchmcm G·n 03.2.3 wAkrbaz Co:II Sarin G.18 03.2.4 Monte ChrisIo· AUll:Iys lUll G-" G3.2.3 Limpopo K=o bin G·21 03.2.6 WQl.... P:ut of WOILiPQon K:In)Q Troulh G-" Gl.2.1 Conc1udinl Remarks G-" 03.3 AJlovU.! Aquifers .... T~ G-D G3.3.1 ~ Ri_ G-D GJ.3.2 Crocodik RIV(1 G·23 03.3.3 M:r.tL1bas RIve' 0·14 G3.3 .• MoI:olo Ri""" 0·24 Gl.)J l.eptW:Ib RiV(1 G-'-5 G3.3.6 MO~!I3RI"" G·2S G).4 AJluvi:l! Aqutfcrs a1on, tllf.ltmpopo Ri,cr G·2S G3A.l IIItrodr.octiol G-" GM.2 $QoR:cs of Inform:tIIOII 0·26 G3.".3 Secrion I: ~e 0 IQ 190 km 0·26 03.4.. Section 2: OWnace 290 1Q~.1O Un G-" GJ . ~ . S Section J: OWnace.5O IQ 470 km G_" G3 .• .6 Sccticn.: OWnace 470 10 SIO km G-" 03.4.1 ~ge from "'" River 0·31 (;).4.B Conclumns;wL Reoommend.uiOtll G·] I CHAP'I"ER 04: SEDIME1."T" YIELD ANALYS IS 04.1 Inl1"Oduct;on G·33 04.1 Rm:trdotd Sediment Yield V:lJues G·33 04.3 Effccti", Dlcttmcnt Sizes 0." 04.4 AYU:Ile VoLt.me t..oaa G·lS 04J ConclllSioots G_" CHAPTER GS: WATER QUALITY SmJATION ASSESSM£r,"f G5. 1 Introduction G·31 m.l Av:ribbiW;y of W~tcr Qu:l.Lity 0313 G·J1 " LIST O ~' CONTENTS (CO,vr4'oIUED) ~eNo m3 Waw- Qu:IJ;1)' Ouide~ne Co:ICtn!ntionl G .. 37 053.L Domestic: Use G-" 0$3.2 AancuJlUr.II Use G-" 0~.).3 lndWilli~ Use G.. 39 05.3.4 ProIccliOll of "''I,,:uic Lit. G"" 05.3.5 R=1ion.>L Use G.O Wilier Qu;lhl)' Assessment Tnbutlries G"" 05".L )o. brico R..vtr G--'O 05.4.2 Cn:l«)dile R..ver G.' 05.4.) NoI~ R.._ G., OS.U ~R.._ G., OS.4.5 Bon~;1S. ~ R LocAne RlYCIJ G4' OH.6 MokoIo ltlve. G4' m.4.7 lcphilib Rlvet G.' 05.4.' M~·.1l3 Riller G.' 05.4.9 MOIIoul.SC Riller G4' G5.5 Walr:r Qu.>Lil)' A$Je$Im<:nc: Limpopo River G4. 05.6 Waler QcuLicy ~ I : Proposetl R."NQII$ G~ OH.. ! Bufi"dsdritc D3m G4' 05.6.2 Sclib D3m G., 05.7 Coox!usions G4' 05.7.1 Tribuwo CHAPTER 06: RESERVOIR YIELDS «-, lmrodudioo1 O·SL G6-2 Ini~ Yield UIl1l3l= G .. '3 G6.J BWcYICLd~ G .. " GO-,' S:u.e CaR Flow Series 0·5' G033 Allem;lllive AI,)w ~uenccs G .. ~, «-. The E.Ustin. System G~' 06.4.1 Yields horn clle Ewe",. System G-6> 06.4..2 ErrCC'! of 03m CorutnlCtiorl G~' 06.. 43 Compens:uion Rcqiji:l:mcnlS G-66 GO.' YieLds from a Multi"lIK 03m 0 .. 61 G6.6 Yields for • MuLci- '" LIST OF CONTENTS (CONTINUED) Page No G6.7 Shapane Offstream Storage G-75 G6.8 Expected Yields for the Future Development Scenario G-77 G6.9 Reliability of Yields in the Early Years G-78 CHAPTER G7: IMPACT ON WATER RESOURCES DOWNSTREAM OF TIiE STUDY AREA G7.1 Impact on Flows at the End of the Study Area G-85 G7.2 Additional Water Resources Downstream of the Study Area G-86 G7.2.1 Shashe River G-86 G7.2.2 Along the Zimbabwe/RSA Border G-87 G7.2.3 Tributaries that Join in Mozambique G-88 G7.3 Impact on the Downstream States G-89 REFERENCES G-91 APPENDIX G-A WATER QUALITY ANALYSIS TECHNIQUES APPENDIX G-B SAMPLE RESULTS OF RESERVOIR YIELD ANALYSIS iv Ll STOF TADU :S ~geNo m.1 An:lIysi. of Boreholes in u... Crooodile·M;uito \..0"'1:\1\11 03.2 Ground".,t ... Qu.>lity in the M3tbba$ Cmclvnent 03.3 Oroundwot ... Qu.>lil)' in the Sllr.ln~ ... .lId 8e~uty An:~ G4.1 SedimcnL Yields Recordtd in Lhe Upper Limpopo B:lSin "'.2 EffectiYe CatclunenL A~ 04,3 Resull'l of Scdjmenl Yield An:lIysis GS.I Wot ... Quality Reconk for RSA SamplinJ $t:ltioru G-38 m.l Woter QwJity Ouidc~nes for Diff~t Use. 0·)9 G>3 F'nIpo$ed Woter QlWity S:lmplinl Requirement$ G·'" 06.1 MAR AAumed fOf PreUmin:lry Assoumcntl of Yield 0·53 0<.2 Prelimin3ly Yield An:lIysis for Limpopo D:Im Site, G·~ 06.3 Summ:.ry of i>:u:Imo:lm for the 83SC ~ Y",1d Assessment O-SS 0<.' l13sc C:t$C Yields for Selected Ibm Si",. G· S6 06.3 Comp:uison Of MAR's fOf Al"'m:lli'e Flow Sequt:nces 0·58 0<.' Comp:uison of Yiekb: for Al",m:ltiw: Flow Sene., [Of the Middle Orovp of SiteS 0·59 Comp:trison 6f Yields (Of Al!em:uiw: Flow Serks I G.., ""06.' Comparison of Yickb: (Of AILem:lL"e Flow Series 2 0·61 06' Eli$ting DeIThlnd Oft the: Upper Limpopo G • UST OF TABLES (COr--rINIJED) 06,27 Reliability of Mtctinll Dcm""" with ~ D:Im 31 Buffelsdrifl SlOring 7SO m'IO' G·go 06.28 Reli3biJity of Mtclinll Dcm.wl ... illl a Dam 31 Rlocnd:llc. Slorinll 7SO m'IO' G·82 0<1.29 Reliability of Mtclin& Dcm.wl willl a Dam at Ri . etSd:tk Sloring 1 000 m'IO' G·82 G6.JO Rell3biUty of 1>I"'linl Oo:m.wl ... illl a Dam 31 SUMyJide Sloring I 000 rn'lO' G·83 G6.31 RetiabililY of M"'lin& Dcm""" with a Dam 31 Selika Storing I 000 rn'IO' G·8J G7.1 MARUpsu=noftheSI\3slIeConn""nce G·8j G1.2 Yi U ST OF F'1G URLS following !':lge No GI,\ C·, 02.1 C4 m.1 G·16 G~.I LoaD()<1 of Wale. Qu31iIY S""'plinJ Points C·" G~.2 0ur;r.1;ion of W:w:. Qu3lilY 0:11.3 Record!; G·3! G~.3 Simul-llcd TDS Coocelllr.ltions for Burr.Udrift Reservoir C'" OS.4 Sim~ TDS COOCC1ltr.llions for Sclib Rc:servoir C4' 06.' Yield:< from Buffelsdnft 0"", C·" C60 Yields from s.1tb !>:un G·S8 C63 Yidd:< from R:IIoo O:un G·S8 06' Dummy D:Im Yield:< Oo...... ,;tr=n of . Limpopo Ihm C-66 06.' Reservoir Simularions for Cumbcrllnd 31 7SO rn'IO' C-68 06.6 Reservoir Simula1ions for Buffelsdrin at 7SO m'IO' C-68 06.' Reservoir Simul:ltion, for Sunnys~ 31 I 000 rn'lIJ' C-68 G6.8 Reservoir Sirnubtions for Sclih all 000 rn'IO' C-68 G6.9 Reservoir Simubtions for Sclik:l at I 000 m'IO' with Buffclsdrin c·n (;(;.10 Yields of SII:tp:me lOiethe. wilh Selit3 G·76 ,- A88 REVI",nONS BNWMPS Boa"""", N:ItJOnlJ W~II:' M:Is." PLvt Study BkGl-t Bwc:au De Rccllcttlles ~oques tI MiNeteS DO Dissolv«! Oxn:en DWA Department of W:Iler Aff:Urs se Elccbic21 COQIucrivily GRES Groundlll.)ter Rcch:u-ie Sv31u:llion SllIdy Ko"! K,e11bhJ NilfOltn LWUS Llmpopo W:w:. U"lis;uior! SlIxly MAP Me:u'I AnII.w PrccipitWon MAR Mc.:lll Annu.3l Runoff r."l1J Neplholomebic Turbidity Unn SABS Soulh African Bu=u of SI:u>d;.nls SAGS Soul/l Afric211 GtolOSicl SUlVey SAlt Sodi .... Adso TDS T oaI D'IS:lol v«! SoIic!s ll' ToW I'!Iosphon.ls TSS TouI Susptllded So~ds USDA Unil.d SUI(.I Dep:utmenl of Agriculrure yi, PREFACE The Joint Upper Lirnpopo Basin Study was undertaken for the Joint Pennanent Technical Committee (JPTC) on Water Affairs of the Republics of Botswana and South Africa. The JPTC was established in 1983, inter alia, . to make recommendations on water apportionment, joint investigations and the co-ordination of water development planning. At a meeting in February 1989, the JPTC agreed that a joint investigation should be conducted to detennine the possible development of the Lirnpopo river. After tenns of reference had been prepared and proposals requested. from a number of consultants, the MacDonald Shand Consortium was appointed in June 1990 to undertake this study. The Study was divided into two stages; Stage I, a pre-feasibility study consisting mainly of a review of previous studies and analysis of available data leading to a recommendation on the most viable development scenario for further study, and Stage II, a feasibility study of the recommended scheme. This Stage I of the study is described in the following reports: Main Report Album of Drawings Annex A: Basin Description Annex B: Soils and Irrigation Potential Annex C: Agriculture and Agricultural Economics Annex D: Dams Studies Annex E: Calibration of Gauging Weirs Annex F : Hydrology Annex G: Water Resources Annex H: Environmental Assessments Annex I Legal Studies Annex J Development Proposals The study has been undertaken mainly in the Gaborone office of Sir M MacDonald and Partners Ltd and in the Pretoria and Cape Town offices of Ninham Shand Inc. In addition, the Consortium has employed a number of sub-consultants to provide input into certain specialist fields. The study was monitored br a Steering Committee, nominated by the JPTC, comprised of representatives of the Departments of Water Affairs of the Governments of Botswana and South Africa. EXECUTIVE SUMMARY Introduction This Annex describes what are essentially five independent studies, namely ground water, sedimentation, water quality, reservoir yields and the effect on downstream countries. These are summarised below. Groundwater In Botswana groundwater continues to play a critical role in the water resources development options proposed in the recent Botswana National Water Master Plan Study. As major villages and other rural centres expand, it will continue to constitute the primary source of supply for many. It is envisaged that groundwater will be used in a conjunctive manner on a national scale as part of the proposed North-South carrier system, to provide carry over supplies during the implementation of surface schemes and periods of prolonged drought. Of particular 2 importance in this regard is the proposed Palla Road welIfield. Extending over 340 km , this aquifer has been 3 6 3 estimated as having a total extractable resource of 167 m 10 for a 40 m draw down and 248 m IQ6 for a 60 m drawdown. The recharge rate is estimated as 2.4 m31~ per annum. In South Africa, groundwater from hard-rock formations appears to be more limited and has no significance as a regional resource, being generally limited to stock watering and rural domestic supplies. Only in isolated areas with more favourable conditions, are small irrigation projects and supplies to lesser urban areas (eg Alldays) possible. Of particular importance to this study are the alluvial aquifers along the Limpopo river. At present these are utilised to a significant extent only downstream of the Motloutse confluence. This study has, however, identified a possibly significant alluvial aquifer in the upstream reaches where there is evidence of ancient river channels ten metres and more below the current river bed, even where rock is outcropping in the current river course. The most promising reach appears to be between the Matlabas and Mahalapswe rivers, where it is speculated that 3 there might be an extractable storage of 100 m IQ6 or more. Depending on the recharge characteristics, this could provide a significant regional resource, and it is recommended that a detailed investigation of this area be undertaken. Sedimentation Sediment yields recorded in both Botswana and South Africa in the Upper Limpopo Basin and surrounding areas, have been collected and studied. The maximum likely sediment yields proposed for the three preferred dams are shown in Table GS.1. These are low by southern African standards, but in view of the very large catchments, could result in significant loss of reservoir storage. GS-l Table GS.l Results of Sediment Yield Analysis Dam site Effective Average Volume loss catchment area unit yield after 50 years 3 (km~ (t/krn2/yr) (m Hf) Buffelsdrift 26409 130 127 Selika 58630 120 260 Ratho 98878 80 293 Water Quality Available records have been collected and analysed to compare the water quality of each of the major tributaries with quality guidelines for different uses. It was concluded that the water is generally suitable for most uses but that from certain tributaries, irrigation of sensitive crops could be problematic. This assessment is based on 1 275 samples in South Africa and 48 in Botswana A mass balance Total Dissolved Salts (1DS) simulation has been undertaken for the proposed reservoirs at Buffelsdrift and Selika. This indicated that TDS concentrations would be highly dependent on reservoir capacity, increasing quite sharply when the reservoir is drawn down. The 50% non-exceedence concentrations are 486 mg/I for Buffelsdrift and 205 mg/I for Selika. The 90% non-exceedence values are 1 258 and 445 mg//, respectively. Water quality at Selika is therefore expected to be significantly better than at Buffelsdrift Considering the paucity of data for the Botswana tributaries, and the distance upstream of most of the sampling points, these results must be considered as very provisional and it is recommended that an intensive sampling programme be put into place as soon as possible, so that better data will be available for Stage II studies. Reservoir Yields Following an initial assessment of the 25 dam sites identified by the reconnaissance studies, yield analysis has concentrated on the 11 sites selected for the pre-feasibility studies. Long-term yield has been estimated for a range of hydrological assumptions, reliability criteria and development options. A 95/99% reliability criterion has been adopted for the assessment of supplies for meeting urban demand, with an 80% criterion used for assessing irrigatiol! :;uppti.es. Due to the indications that the lower three sites, Mopani, Ratho and Ponts Drift. were not viable for meeting urban demand in Botswana, and in any case not desirable on environmental grounds, a full yield analysis has not been undertaken for these sites. An assessment has been made of current yield from the numerous irrigation weirs along the Upper Limpopo, on the basis of an assumed demand relating to the 1981 level of irrigation (the highest on record) and a 150% 3 cropping pattern. Only 40% of the 245 m IQ6 demand was met, confrrming that the assumed levels of irrigation are not sustainable in the long-term. The reduction in yield from weirs downstream of a Limpopo dam was also assessed, to enable the level of compensation releases to be established in order to maintain current levels of supply. Due allowance has been made for transmission losses when deriving these compensation releases, which are summarised in Table GS.2. GS-2 Table GS.2 Compensation Releases Compensation releases (m310~ Dam site RSA Botswana Total CumberIand 102 2.5 12.7 Buffelsdrift 12.8 6.2 19.0 Riversdale 12.2 5.4 17.6 Sunnyside 10.2 1.1 11.3 Selika 29.4 2.5 31.9 Martins Drift 21.3 1.6 22.9 Worcester 15.4 1.0 16.4 Graaf Reinet 9.4 0.6 10.0 Estimates have also been made for the yield of the current system under different time-horizons. Water resource developments in the decade of the eighties have resulted in a reduction in water for irrigation of approximately 10%, and, it is predicted that farmers will be further disadvantaged by a reduction of an additional 15% by the year 2020. A Limpopo Dam could help to alleviate this problem. Yields have been assessed for a multiple use of the reservoir, with some supplies to meet urban demand and releases also being made for irrigation use. A number of different demand patterns have been evaluated, with the base case taken as a 50/50% split between RSA and Botswana, with all RSA's share going to irrigation and all Botswana's portion going to meet urban demand, after due allowance for compensation releases for existing irrigators in Botswana. The results are presented in Table GS.3. The yields from the Middle Group of sites have also been established for the situation where a dam had been previously constructed at Buffelsdrift (capacity 750 m31O~, with results presented in Table GS.4. An assessment has been made of the potential of an offstrefun storage site, at which improved storage characteristics result in lower evaporation losses. The proposal is based on a transfer of half the water from Selika dam to an offstream storage site at Shapane. Consideration has been given to a range of sizes for both Selika and Shapane and for the transfer capacity between the two. Significant incremental yields are possible, 6 3 6 for example the 95/99% yield from a Selika dam storing 750 m310 can be increased from 23 m 10 /yr to 36 m3106/yr with aShapane storage of 250 m3106 and a pumping capacity of 100 m31~/yr (3.2 m3/s): both yields are based on a 50/50% split but make no allowance for compensation releases or the reduction in yields inherent in multi-use operation. Yields have also been established for Limpopo dams under a "future development" scenario, whereby the hydrology has been revised to reflect a pattern of water use prevailing in 2020. Yields arereduced by between 25% and 50%, with an average of about 35%: reductions are generally in line with reductions in MAR's at the sites. GS-3 Table GS.3 Joint Yields for 50/50% Share Yield at stated reliability (m 3l06/yr) 3 6 Dam site Reservoir storage (m l0 ) 250 500 750 1 000 1250 1 500 2000 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% Cumberland 15.7 1.5 26.3 5.3 37.1 tO.1 41.3 12.2 45.5 12.6 53.9 13.4 59.0 13.7 Buffelsdrift 34.9 4.8 40.1 8.2 52.1 13.6 62.0 17.6 65.0 17.3 78.1 19.6 81.4 18.9 Riversdale 34.3 "5.6 48.9 11.3 54.7 16.2 67.1 23.6 73.9 24.5 76.3 24.1 94.2 26.6 Sunnyside 23.2 5.2 40.7 10.8 56.5 18.5 65.4 24.3 80.7 31.7 81.1 " 35.6 99.1 35.3 Selika 39.2 6.5 49.6 11.6 60.6 18.1 74.5 24.5 83.3 28.0 93.9 32.7 114.5 39.5 Martins Drift 40.1 7.2 52.4 11.6 58.9 18.4 73.4 24.7 81.4 27.8 89.1 33.0 109.4 39.2 Cl Worcester 40.9 9.2 55.4 15.9 63.0 22.6 81.1 29.7 87.8 34.9 100.0 39.3 117.3 47.0 CI.l J,.. Graaf Reinet 27.9 7.6 46.9 17.0 62.8 25.4 75.8 30.8 90.5 37.8 103.0 42.3 129.9 48.2 ------,-- - Table GS.4 Joint Yields with a Dam at Buffelsdrift for a 50/50% Share Yield at stated reliability (m 3 t06/yr) 3 6 Dam site Reservoir storage (m 10 ) 250 500 750 1 000 1 250 1 500 2000 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% Sunnyside 16.9 2.7 26.0 4.3 37.3 6.5 43.6 8.8 50.9 9.1 59.1 9.5 57.4 9.9 Selika 26.5 5.4 38.9 8.8 51.6 11.4 59.6 13.4 68.8 14.9 73.2 15.6 87.4 16.0 Martins Drift 27.1 6.1 38.3 9.6 51.2 12.0 59.1 13.8 65.2 16.0 71.6 16.1 76.1 15.9 Worcester 29.3 8.4 43.2 12.4 56.7 15.5 66.3 17.8 73.6 20.4 77.3 20.7 87.3 20.5 Graaf Reinet 20.5 5.2 35.4 8.6 48.2 11.5 59.3 14.9 65.9 17.5 80.3 19.2 104.7 21.3 L..-______The final factor to be considered in this aspect of the study is requirement for fIlling time for the reservoirs in order to guarantee yields at the required reliability. This is a complex subject and it has only been possible to make a preliminary analysis. Five of the most promising sites have been assessed, resulting in the following recommendations for the time to be allowed for impounding: Cumberland over four years Buffelsdrift three years Riversdale two years Sunnyside four years Selika two years Impact on the Downstream States An assessment has been made of the impact of a darn on the Upper Limpopo on flows below the Shashe confluence, where the river constitutes the border between Zimbabwe and RSA, and into Mozambique, where the Limpopo flows into the Indian Ocean. Indications are that the difference in flows due to the darn will be comparatively small, perhaps only a 10% reduction in MAR at Beit Bridge, which is about 70 km below the Shashe confluence. However, what appears to be of greater significance is the general level of water resource development throughout the catchment of the Limpopo. There remains considerable doubt about the flow losses in the lower portion of the study area. It is, therefore, recommended that a gauging station be established as close as is practical to upstream of the Shashe confluence. Reliable information from this location will prove invaluable for any discussions between the four countries that share the basin. Such discussions will undoubtedly be required at some future date, given the inevitable trend to harness greater proportions of the region's water resources. GS-5 CHAPTER Gl INTRODUCTION Gl.l GENERAL This report describes the water resources of. and adjacent to. the Limpopo river. from its source at the Marico/Crocodile confluence to the confluence of the Shashe river. some 500 km downstream. The locations of twenty-five possible dam sites identified along this stretch are shown on Figure Gl.l. Chapters G2 and G3 describe the groundwater resources of Botswana and South Africa respectively. Each country was studied by an expert familiar with the groundwater of that country and. because of the difference in the availability of infonnation and the differences of approach usually adopted in each country, the two studies are reported separately. Chapter G4 describes the sediment yield analysis undertaken to predict the potential sediment accumulation at the different dam sites. and Chapter G5 describes an analysis of the water quality of the Limpopo river and the simulations of water quality of the proposed reservoirs. Chapter G6 covers the assessment of yields that has been undertaken for the selected dam sites. This chapter also includes the estimates of yields from the existing system of irrigation weirs. Chapter G7 considers the possible impact of a dam on the Upper Limpopo on the water resources lower down the river. in particular with regard to existing and potential developments in Zimbabwe and Mozambique and the portion of RSA below the Shashe confluence. G-l Figure G 1.1 Location of Preliminary Dam Sites tse MOt/Ou °Sobonong Drift o :ooane 58diOtl 0 Mmadinare Tnune SeJebj-Phj~we 0 Bains Drift o Sefophe Marapong Bains Drift Dunsandle Bambata Benedict Geluk REPUBLIC OF BOTSWANA OMarnitz LEBOWA !~ I Marakalalo LEBO~ S Halt REPUBLIC OF SOUTH AFRICA Matlabas LEGEND ~ DAM SITES SCALE 10000m Okm 10 20 30 40 50km CHAPTER G2 BOTSWANA GROUNDWATER STUDY G2.1 INTRODUCTION As part of the overall assessment of the water resources of the Upper Limpopo Basin it is necessary to examine the groundwater potential of the geological strata which underlie the basin both as possible sources of water supply in their own right and in terms of their hydrogeological influence on any surface water development scenarios which may be proposed. A brief overview of the hydro geological regime in the Upper Limpopo Basin is thus presented, together with a review of significant existing ground water developments and considered comment on other potential groundwater resources. Many of the place names referred to in the text have been included in a general locality map, Figure G2.1. Other specific hydro geological factors which may impinge directly upon the evaluation of possible Limpopo development options are also appraised. G2.2 HYDROGEOLOGICAL ENVIRONMENT Examination of the geological fabric of the Upper Limpopo Basin area (Album Drawing No MSC 1/109) indicates that the basin is dominated by lithological units which lack primary interstitial permeability either as a result of the metamorphism of their original sediments or by the intrusive or volcanic nature of their genesis. Only in the north, east and west central regions are younger (Karoo) stratiform sediments, which potentially form primary aquifers, present. Elsewhere the northern portion of the basin is predominantly underlain by granitic and high grade metamorphic Archean gneissic formations, with the southern regions of the basin occupied by Proterozoic metasedimentary units of the Transvaal Basin and surrounds. It is thus apparent that the study area is largely one of secondary aquifers, characterised by a lack of hydrogeologiCal uniformity which in turn results in a general reduction in the predictability of groundwater potential. This in particular relates to the actual likelihood of groundwater occurrence and the magnitude and longevity of individual borehole yields which may be expected from individual sites. The relative predictability of the different types of aquifer, and hence the degree of confidence with which the groundwater resources may be estimated, varies considerably. Primary aquifers in general allow the extrapolation of resource evaluation from one area to another, while secondary aquifers which contain ground water bodies which are both difficult to locate and difficult to quantify do not lend themselves to such extrapolation. In this latter case, therefore, resource estimates may be both problematical and lacking in reliability, a factor which must be borne in mind when appraising groundwater potential in a broad basin-wide manner. One of the more positive features of the aquifers in the Upper Limpopo Basin is their relatively good potential for replenishment The recharge of groundwater from precipitation and run-off is greatly enhanced, in general, by a reduced superficial cover over the main aquiferous zones. In the case of the Upper Limpopo Basin, superficial deposits are fairly thin, except perhaps in the west central area on the fringes of the Botswana Kalahari where sand cover may reach up to 40. metres in thickness. Recharge to the groundwater body is thus fairly rapid and relatively frequent, although subject to the same drought cycles which so significantly affect surface water sources. The great advantage of a groundwater source is, however, that it is not subject to G-3 significant losses by evaporation and hence may provide considerably more security of supply in comparison with surface water sources. This is an important factor when considering groundwater and surface water sources in a conjunctive use scenario. G2.3 SIGNIFICANT AQUIFERS Within the confmes of the Upper Limpopo Basin there are several geological units which may be regarded as possessing aquifer properties of regional importance. Each such formation is discussed briefly below. G2.3.1 Waterberg Formations These formations comprise a series of sandstones, shales, siltstones and conglomerates of Proterozoic age, all partially metamorphosed, possessing little or no intrinsic permeability and constituting only secondary aquifers. Waterberg strata occur in two principal areas within the basin. To the south they occur in a broad belt trending ENE/WSW through the major villages of Molepolole and Mochudi. Strata are folded into a wide syncline and are frequently intruded by dolerite dykes and sills and are highly fractured. In this area the lowermost sandstone unit appears to be the most prolific aquifer. Average borehole yield from Waterberg strata is 4.1 m3Jhr (1.1 lis) (Botswana National Water Master Plan, 1990) and average success rate of boreholes drilled is some 68%. Groundwater from Waterberg strata is utilized for the supply of both Mochudi and Molepolole (see Section G2.4 below) and resources are being further investigated and developed (see Section G2.5). Waterberg formations also occur in the north-central part of the basin in the vicinity ofPalapye (palapye Group). Here they constitute the elevated areas of the Tswapong Hills and comprise a sequence of dolomitic limestones, sandstones, quartzites and siltstones transected by innumerable dolerite dykes. Average yields are around 7 m3Jhr (21/s) with an average borehole success rate of some 85% from the water bearing formations. Palapye Group aquifers are exploited for the supply of Palapye (see Section G2.4) and other smaller centres in the vicinity. G2.3.2 Transvaal Formations The principal aquiferous strata in this unit are the dolomites and quartzites which occur in a narrow band trending N/S between Lobatse and Ramotswa. This band forms the easternmost extension of the very large Transvaal basin which occurs in RSA. These strata are both secondary aquifers, with the dolomite units possessing very large secondary permeability in the form of karstic developments in an area north of Lobatse and in the vicinity of Ramotswa. The presence of karstification renders the dolomite strata an extremely prolific aquifer having excellent transmission and storage properties and very good recharge potential. Transvaal aquifers are developed for urban supply at Lobatse, Ramotswa and. to a lesser degree, at Kanye (see Section G2.4). G2.3.3 Karoo Formations Strata belonging to the Mesozoic Karoo Supergroup are recognised as the most significant aquifers in Botswana, but as a result of geological and geomorphological history they generally occur only on the western margin of G-4 Figure G2.1 Locality Plan: Botswana Groundwater o Francistown Serowe Botlhapatlou SCALE Okm 100km the Upper Limpopo Basin on the fringes of the Kalahari. Exceptions to this include an area to the south of Mahalapye, around Mmamabula and Palla Road, where a number of large blocks of fault controlled Karoo strata exist, and an area to the northeast in the vicinity of Bobonong where a large Karoo synclinal structure occurs. This latter forms the western termination of the large (Wankie) Km:oo basin of southern Zimbabwe. The Karoo Supergroup comprises a thick sedimentary sequence of sandstones, shales, coals and mudstones of considerable lateral and vertical variability (Ecca) overlain by a very widespread and relatively uniform aeolian sandstone unit (Ntane) capped by a thick series of basaltic lavas (Stormberg). The principal aquifer units are the middle Ecca sandstones (variously named the Kweneng, Mmamabula, Mea Arkose Formations) and the aeolian Ntane sandstone. In hydrogeologically favourable environments the former provide very high individual bore hole yields and are developed at a number of localities (especially Jwaneng Northern Wellfields) for water supply. The Ntane sandstone constitutes a more homogenous aquifer of greater areal consistency but lower bore hole yield and is developed for major supplies at Orapa and Serowe. Extensive investigations relating to the distribution and groundwater potential of the Karoo strata have been undertaken and to date they constitute the best known aquiferous formations in the country. G2.3.4 Sand River and Alluvium Several of the larger Botswana tributaries of the Limpopo (Shashe/MotIoutse/Mahalapswe) contain significant sand river aquifers. The resources of these elongate highly transmissive aquifers have been studied under the Sand River Project of the Department of Water Affairs (Wikner, 1980; Nord 1985). Sand thickness is of the order of 5 to 10 metres and the sediments usually comprise medium to coarse grained sand with lesser, but usually equal, amounts of gravel and fine grained sand/silt. Storage properties are excellent, with an average storage coefficient of 17.5%, and transmissivities range from 400 to 1 500 m2/d (MacDonald, 1987). The total recharge to the sand river aquifer occurs with every river flow. The aquifers are, however, very susceptible to pollution. Alluvial aquifers also occur along the Limpopo channel, but appear to be especially well developed at the confluence of the tributaries and the Limpopo itself. Along the Limpopo they comprise fine sand and silt but in the confluence areas of the Motloutse and the Mahalapswe the deposits are thicker and coarser, containing a much greater proportion of medium coarse sand and gravels. Both these latter areas are underlain by Karoo strata and the alluvial deposits are thought to have a hydraulic connection with them. G2.4 GROUNDWATER RESOURCES: CURRENT SITUATION G2.4.1 General Groundwater has been and continues to be a vital component of all aspects of national development. Historically it has formed the basis of development of the cattle industry and, in the absence of any significant surface sources, has formed the primary source of water for traditional centres of settlement such as Lobatse, Kanye, Serowe, Palapye and Francistown. G-5 With the move towards independence and the establishment of Gaborone as the new capital some 20 years ago, the need to supply larger urban populations resulted in the construction of both the Nnywane (Lobatse) and Gaborone Dams. The availability of water from such sources enabled rapid urban expansion and lessened dependence on groundwater. Similarly in Francistown with the construction of the Shashe Dam. Only in centres where no surface potential existed did groundwater demand continue to escalate with population rapidly becoming increasingly urbanised. Small wellfields, or interlinked borehole sources, were thus installed in centres such as Kanye, Mahalapye, Palapye and Serowe and an increasing number of boreholes drilled to supply many other smaller villages in the basin. The continued development of the cattle industry also led to the establishment of innumerable watering points owned by individuals or cattle owning syndicates. The growth in urban demand for water, which has been evident in the last few years, and is expected to continue to increase rapidly, has prompted a number of studies into the potential for an integrated supply network drawing on both surface and groundwater resources; the latest of these being the Botswana National Water Master Plan Study (BNWMPS, 1990). These general studies have formed the framework for a number of investigations into the potential of specific aquifers. The most up-to-date picture of the potential of these groundwater resources and their current level of development can best be outlined with reference to the various demand centres to which they are linked. G2.4.2 . Transvaal Formations - Lobatse Area Four different aquifers, all comprising Transvaal formations (primarily dolomite), have been investigated and developed over a long period of time in the Lobatse area. Their groundwater potential has been reassessed on a fairly regular basis as a result of rapidly escalating demand and failure of surface supplies (Davies, 1980; Bons and van Loon, 1985; De Vries, 1985; Gibb, 1986; MacDonald, 1987) and a considerable amount of research work has been applied to the reliable evaluation of natural recharge to these aquifers (Gieske et al 1989, 1990). In the case of these latter studies, work is currently ongoing. Each of the four Lobatse aquifer areas (basins) are summarised below in relation to their hydrogeological characteristics and resource potential. (i) Township Basin: Karstic Transvaal dolomite aquifer recharged by hillslope runoff. Total storage 1.87 m3IQ6 Renewable storage 0.56 m2106 (BNWMPS, 1990) (Recharge + lateral flow) Aquifer area 8.0 m21~ Basin area 16.53 m2IQ6 Normal abstraction 0.32 m3IQ6/yr (Gibb, 1988) Maximum abstraction 0.56 m3106/yr (Gibb, 1988) G-6 (ii) Woodlands Basin: Transvaal quartzite two layer aquifer system recharged from the influent Peleng River. Total storage 1.25 m3106 Renewable storage (recharge) 0.20 m31Q6 (BNWMPS, 1990) Aquifer area 9.77 m21Q6 Normal abstraction 0.18 m3106/yr (Gibb, 1988) Maximum abstraction 0.216 m3106/yr (Gibb, 1988) (iii) Pitsanyane Basin: Transvaal dolomite karstic aquifer with principal resources along major structural feature. Possible two levels of karst. Recharge by hillslope runoff and streamflow. Total storage 3.8 m3106 Renewable storage 0.72 m31Q6 (Recharge + lateral flow) Aquifer area 17.0 m21Q6 (BNWMPS, 1990) Recoverable volume 2.0 m31~ Normal abstraction 0.56 m31Q6/yr (Gibb, 1988) Maximum abstraction 0.672 m3IQ6/yr (Gibb, 1988) (iv) Nnywane Basin: Transvaal dolomite aquifer with karstic zones and high transmissivities. Recharge from basin slopes and direct infIltration. Total storage 2.81 m3IQ6 Renewable storage 0.19 m31Q6 (BNWMPS, 1990) (Recharge + lateral flow) Aquifer area 2.5 m21~ Recoverable volume 1.4 m31~ Normal abstraction 0.12 m3106/yr (Gibb, 1988) Maximum abstraction 0.672 m3106/yr (Gibb, 1988) Following substantial rehabilitation works ofpre-existing groundwater sources in the Township and Woodlands Basin and additional exploration and development in the Nnywane and Pitsanyane Basins, it has been established that the optimum one in twenty year system yield of the Transvaal aquifers of the Lobatse area is 327 m3IQ6 per annum (Gibb, 1988). Continued assessment of the results of the ongoing Groundwater Recharge Evaluation Study (GRES) has, however, recently indicated (Gieske, pers. comm) that the total storage volume of the Pitsanyane Basin may be substantially greater than previously thought, with a consequent influence on long-term resource yields. G-7 Present resource development in the Lobatse aquifers involves the installation of equipment and pipelines to enable the above system yield to be achieved, since current surface supplies are inadequate. G2.4.3 Transvaal Formations - Ramotswa Area Dolomites and shales of the Transvaal age constitute a prolific aquiferous area at Ramotswa, some 25 km south of Gaborone. Since its original identification in 1979, this resource has been extensively studied and gradually developed to provide a source to supplement the surface water supply of Gaborone. The dolomite aquifer has both a shallow and deeper zone of karst developed along linear fault features in the dolomite. The shale aquifer also appears to have an upper and lower fissure zone but these may reflect borehole intersection of a series of subvertical fault planes. Both aquifers are essentially linear features with high transmissive and storage characteristics along their centreline which rapidly decrease away from the fracture centres. Definition of the hydro geological boundaries of the aquiferous area has proved difficult, particularly to the east where it is thought to occur some 10 km east of the Notwane river. Since the mid-1980's a number of studies have been undertaken to examine the hydrogeology of the area, to locate additional high yielding boreholes, to detennine pollution potential and to establish a data base and ground water model (BRGM 1985; WLPU 1985; Selaolo 1985, 1986; IoR 1986; WLPU 1989, 1990). Estimates of total resource volumes have been calculated as 18 m31 Further evaluation of the Ramotswa aquifers is currently ongoing and involves long-tenn testing of groups of boreholes, continued water level and hydrochemical monitoring and recharge studies, and the refinement of a computer based aquifer model. G2.4.4 Transvaal Formations - Kanye Area 2 An area some 4 km south of Kanye is underlain by Transvaal dolomites which extend over some 360 km • The region has been examined and resource estimates produced during the course of two studies (BRGM, 1986, 1988). Karstification of the dolomites is limited, being largely restricted to shallow zones often above the regional watertable and generally absent in the southern part of the area Major shear zones play a great role in enhancing the overall aquifer potential. An average transmissivity for the whole aquifer of 10-30 m2/d has been calculated, with much higher values in karstic or fracture zones. Modelling has indicated a total recoverable (mining) resource of some 3.7 m31 G-8 G2.4.S Waterberg Formation - Mochudi and Molepolole Metasediments and dolerite intrusives, of Waterberg age, extend in a broad arc from the Limpopo valley in the east via Mochudi to Molepolole in the west Both major villages are supplied from small wellfield developments in structurally controlled hydrogeological environments in the lower Waterberg sandstones. A number of investigations related to increased supplies for both villages have been undertaken (B uckley, 1984; BRGM, 1986; Keller, 1988). Individual borehole yields may be high but in general the Waterberg aquifers are characterised by poor storage properties. For the whole of the Waterberg area the following resource estimates have been compiled (BNWMPS, 1990). Total storage 211.7 m3106 Recoverable storage 65.4 m3106 Catchment area 3749 m2106 Recharge 54.4 m3106/yr Recharge studies have also been undertaken (Sloots and Wijnen, 1990) in the Waterberg area to the northeast of Molepolole, and show large variations in recharge quantities dependent on the assessment method applied. However, it would appear that a recharge value of 3 to 4% of MAP would be most appropriate. G2.4.6 Waterberg Formations - Palapye Area Waterberg strata occupy an area of some 1 900 km2 between Palapye and Moeng and give rise to the Tswapong Hills. Principal aquifers are the Tswapong and Selika quartzites and metasandstones. These units have been investigated for their potential to supply Palapye (Neumann-Redlin, 1984) and Moeng (Katai, 1989) and a wellfield to provide water to Palapye has now been developed. Significant recharge occurs over the Tswapong Hills, with a volume of 0.8 m3106/yr being estimated for the 6 km2 catchment of the Palapye wellfield (BNWMPS, 1990). For the Tswapong aquifer over the whole of the area a recoverable resource of 9.6 m3 106 and an annual recharge of 13 m3106 has also been estimated (BNWMPS, 1990). No estimates of the resource potential of other, smaller more isolated areas of Waterberg strata, which occur in the central portion of the Upper Limpopo Basin, have been made. G2.4.7 Karoo Formation - Mmamabula Area Both the Ntane sandstones and the Ecca formations OCcur in an east-west block to the south of Mahalapye in the Palla Road - Mmarnabula area. At Palla Road the Ntane sandstone aquifer has been investigated and developed to provide a water supply to Mahalapye. The median yield for individual boreholes in the area is high, at about 30 m3/hr (8 lIs), considerably in excess of that in the Serowe area and comparable with the highly productive Ntane aquifer at Orapa. This is thought to be as a result of the influence of block faulting and hence a general increase in fracture permeability in the area (BNWMPS, 1990). The current wellfield is also adjacent to the Serorome Valley which may be a structural feature controlling regional groundwater flow. Recent studies (BNWMPS, 1990) have assessed the resource. potential of a Palla Road wellfield extended to the west and 2 northwest to occupy an area of 340 km • Allowing a 40 metre drawdown the total extractable resource is estimated as 167 m3106• For a 60 metre drawdown this value is increased to 248 m31~. Total replenishment from G-9 lateral inflow and from direct recharge is estimated as 2.4 m3 IQ6Jyr. In the same Palla Road/Mmamabula area, Ecca aquifers are also present at depth below the Ntane sandstones and at subcrop. An initial study of the whole area (Aquatech, 1988) identified two areas as having good groundwater potential (Khurutse and Zoefontein) from both the upper Karoo Ntane aquifer and the lower Ecca aquifers. Initial estimates (Aquatech, 1988) of recoverable storage were 410 m3 1(f and 450 m3 1Q6 respectively. However, more recent re-examination of the area (BNWMPS, 1990) proposes much reduced recoverable (mining) resources. Estimates for the combined Karoo aquifers (Ntane, multiIayer Ecca) over the whole Mmamabula area (including PalIa Road), have been made for two sets of assumptions. The storage value of the Ntane sandstone is varied from 1% (Scenario 1) to 3% (Scenario 2), with higher transmissivity values adopted for the Scenario 2. Scenario 1: Khurutse 31.1 m3IQ6 Central Mmamabula 45.7 m3 IQ6 Palla Road 24.0 m3106 Zoetfontein 30.5 m3106 Total extractable volume Scenario 2: Khurutse 69.4 m3106 Central Mmamabula 104.2 m3106 Palla Road 522 m3106 Zoetfontein 72.6 m3106 Total extractable volume G2.4.8 Sand Rivers - Lower Sbasbe, Motloutse and Mabalapswe J1le Lower Shashe, Motloutse and Mahalapswe are all categorised as Major Sand Rivers (Wikner, 1980) and have theoretical recoverable volumes of groundwater in storage in excess of 100 m3/dIkm (1 l/s/km). Several water supply schemes based on sand river aquifers are operational (Mahalapye, Mmadinare, Tonota-Shashe) and utilize screened fIlter wells or well point systems. Elsewhere the sand river resources are widely used for local supplies and cattle watering. G2.4.9 Alluvial Deposits - Talana and Dikglatong Areas At the confluence of the Motloutse and the Limpopo (Talana) a prolific alluvial aquifercovering an area of some 15-20 km2 and some 5-20 m thick has been inv~stigated and developed for irrigation water supply. The volume 3 of the main aquifer at the confluence is estimated as 84 m IQ6 and has an average transmissivity of 2 700 m'1/d (Aquatech, 1983). The Talana aquifer is divided into two segments by a dolerite dyke (Solomons Wall) crossing G-lO the Motloutse some 4 km upstream of the Limpopo confluence. Total storage volume in each segment is 3 estimated as 3.27 and 5.18 m IQ6 respectively (assuming a storage coefficient of 3.5%), with a recoverable 3 6 volume of 1.7 m 10 being calculated for the latter (upstream) segment (Aquatech, 1983). Current annual abstraction is estimated as 2 m3106 (BNWMPS, 1990). The alluvial aquifers are replenished by river flow in the Motloutse and the Limpopo, areal precipitation and most probably by discharge from the underlying Karoo (Ntane) sandstone. A similar alluvial aquifer situation pertains at Dikglatong at the confluence of the Mahalapswe and the Limpopo where a faulted block of Karoo strata is overlain by an alluvium sequence of gravels, coarse sands and superficial silty sand and marls. The aquifer is assumed to have a thickness of some 15-20 metres and to be in hydraulic connection with the Karoo (Ecca) sandstones. With an assumed storage of 20% a total recoverable 3 6 resource of 30 m 10 and an inflow of 0.6 m3106/yr from the Mahalapswe river area has been calculated (De Vries, 1985). Aquifer transmissivity is very high at 1 500 m2/d. G2.S GROUNDWATER RESOURCES: FUTURE POTENTIAL G2.S.1 General Groundwater continues to play a critical role in the natural water development options promulgated in the recent Botswana National Water Master Plan Study. As major villages and other rural centres expand, it will continue to constitute the primary source of supply for many. Groundwater will also be utilized in a conjunctive manner on a national scale as part of the proposed North-South Water Carrier system both to provide carry-over supplies during the implementation of surface schemes and during periods of drought. Potential groundwater resources thus continue to be investigated and evaluated. Although several of these potential resources fall in the Karoo areas to the west, outside the limits of the Limpopo basin, any major supply developments will be intended for demand centres in the basin and they may thus have considerable influence on water development strategies within the Limpopo Basin area. Such potential resource areas, together with others within the basin proper, will thus be commented on below. G2.S.2 Karoo Formations West of Serowe Several studies (Neumann-Redlin and Selcwale, 1982; SGAB, 1988) have examined the groundwater potential of an extensive area of Ntane sandstone (overIain by Stormberg basalts) to the west and northwest of Serowe. The sandstone aquifer is of the order of 100 - 130 m thick, is extensively block faulted into discrete compartments which are generally hydraulically isolated, has substantial storage potential but relatively limited transmissive properties and is recharged along it eastern margins. The relatively homogenous nature of the aquifer and its huge areal extent over most of the Central Kalahari Basin provide for very large volumes of water in storage. (250 000 m31~ excluding possible saline areas - BNWMPS, 1990). However, significant resource development would require a very extensive wellfield infrastructure of relatively low yielding boreholes in order to tap the whole resource. In the area immediately to the west of Serowe (1 380 km~ a total storage of 6 3 6 2 200 m3 10 , a recoverable (mining) storage 6£.66.7 m 10 and an annual replenishment of 2.8 m31~ have been calculated (BNWMPS, 1990). Groundwater from these aquifers is currently utilized to supply Serowe and the nearby power station at Morupule. G-ll G2.S.3 Karoo Formations in the Letlhakeng - Botlhapatlou Area An area of Karoo (Ecca) strata on the margins of the Central Karoo Basin, some 40 lci10metres northeast of Gaborone, was suggested by VIAK (1984) as having good groundwater potential because of its many similarities with the Jwaneng Northern Wellfield area. Subsequent studies initially did not reveal such potential (Bucldey, 1984) but latterly have indicated very high potential from Ecca sandstones in a similar depositional environment as the prolific Jwaneng wellfield (Wes, 1990). Additional resource evaluation studies are nOw under way in adjacent areas and initial results appear to confmn high groundwater potential (BRGM, 1990), but to date no resource estimates are available from the study, which is still in progress. G2.S.4 Karoo Formations - Mmamabula Area Karoo formations of the Mmamabula region form one of only two areas of Karoo strata which occupy a significant portion of the Upper Limpopo Basin proper and which are in direct continuity with similar strata across the Limpopo in RSA. Their resource potential as currently estimated is discussed in Section G2.4.7 above. However, since they are of great importance from the point of view of their apparent resource volumes and their location, additional detailed evaluation studies are to be commissioned during 1991-92 to provide resource estimates of improved reliability as a vital first step in their development G2.S.5 Karoo Formations - Tuli Syncline Area This large area of Karoo strata is located in the region of the Limpopo - Shashe confluence and is contiguous with the Karoo formations of the Wankie Basin in Zimbabwe. Only limited data exist and the area is poorly known hydrogeologically. The region comprises Stormberg basalts overlying Ntane sandstone with an area of 2 some 3 700 km • Recharge prospects are good as a result of basin boundary structures and the proximity of major river sources. A number of estimates of groundwater potential have been made. VIAK (1984) estimates the recoverable resource as 9.5 m3106/yr (300 llsec), MacDonald (1987) as 15.8 m3106/yr (500 l/sec) and the BNWMPS (1990) a total resource of 5920 m3106 with a recpverable (mining) resource of 65 m3 1Q6/yr (2 000 lIs): BNWMPS estimates the recharge to be 15 m3106/yr.. It is apparent that the Tull SyncIine area possesses considerable groundwater potential but, to date, its distance from possible demand centres has afforded the area only low priority for additional investigations. However, the potential of a Limpopo dam on the lower reaches of the river raises the possibility of a groundwater development to be used conjunctively. Should these options be considered further then additional investigations would be required. G2.6 HYDROGEOLOGICAL CONSIDERATIONS RELATED TO A LIMPOPO DAM A number of particular hydrogeological factors should be considered in relation to the selection of any potential dam sites of the main stem of the Limpopo. The more significant of these are discussed below. G-12 G2.6.1 River Flow Losses to Adjacent Aquifers A river may be either influent or effluent in relation to contiguous riverine aquifer by virtue of the relative level of the groundwater table and the surface flow, assuming definite hydraulic connection between the two. For the river to be influent the piezometric level of the river must be higher than that of the groundwater contained in the aquifer, and as a consequence water will be lost from the surface water by flow to the groundwater body. If such losses are significant they will be detectable by flow measurement (gauging) of the river at various points. However, in the absence of adequate gauging on appropriate sections of the river, some assumptions can be made regarding potential seepage losses (forming part of the aquifer recharge) with reference to prevailing hydrogeological conditions. The locations of the 25 dam sites identified in the reconnaissance studies (see Annex D) are given in Figure G 1.1. Several significant aquiferous formations are crossed by the Limpopo. They are the Waterberg strata between OIifants Drift and Riversdale, the Karoo strata between Riversdale and Holm lea and the Karoo units in the vicinity of the MotIoutse - Limpopo confluence. Other minor, but locally important, aquifers strata are found in the fractures running parallel to the amphibolites intrusions in the Seleka/Martins Drift area and the alluvial aquifers at several localities along the channel. In this latter case groundwater recharge is probably by direct infIltration from the river with consequent stream flow losses during the flow periods and possible gradient reversal and ground water discharge during the dry season. In the area of the amphibolites around Seleka Farm it would appear that the Limpopo is not influent since the river levels are some 2-4 metres below the adjacent groundwater table (Gitec, 1983). Elsewhere along the Limpopo regional groundwater gradients appear to either parallel the river or indicate flow towards the river. However, in the absence of any detailed piezometry near the river it is possible that river flow losses may occur into both the Waterberg and Karoo strata, most especially in situations where sandstone aquifers are transected by major fracture zones and/or intrusive dykes which create fracturing. The likelihood of groundwater recharge (flow losses) is further enhanced if such features are provided with a longer term storage and recharge potential by the presence of overlying river-situated alluvial deposits. These conditions may pertain at the junction of the Zoetfontein and Mahalapswe Faults at the confluence of the Mahalapswe and Limpopo Rivers and at other similar locations along the Karoo and Waterberg sections of the river. G2.6.2 Areal Relationship Between Aquifers and Potential Dam Sites Discussion in this context must, of necessity, be very site specific involving area of inundation, recharge potential related to hydraulic head provided by the water body, reduction in accessible (and hence extractable) groundwater resources, groundwaterlsurface water quality interaction and alternative ground water sources. Only at two potential dam sites presently under consideration do these factors warrant serious consideration. These are Ratho and Ponts Drift, downstream of the Motloutse/Limpopo confluence and the prolific Talana alluvial aquifer (see Section G2.4.9). Construction of a dam at this site would inundate the TaIana aquifer and the irrigation areas situated on and adjacent to it This would preclude the further utilization of the aquifer but would most probably considerably enhance recharge to the underlying Ntane sandstone. Since the Ntane sandstone occurs over extensive areas to the north and northwest of J'alana (albeit below Stormberg basalt) this improved recharge could potentially be intercepted by deeper boreholes if a ground water development programme was envisaged. G-13 ) G-14 Figure G3.1 Locality Plan: South African Groundwater X\ SCALE ~m ~ ro~m ~~ LEGEND SU8REGION PRINC:P.~L ROCK 7YP"S I limpopo !owiand A CroccdlIe - Marico Lowiand Granitic and -=fystaliine metamorphic rocks B Matlabas Catchment Sandstone and mudstone C Waterberg Coal basin Sandstone. slltstcne. shale, mudstone. basalt D Monte Cnr!sto - Alldays Granitic and crystalline metamorphic rocks E Umpopo Karoo basin Sandstone, siltstone, shale, mudstone, basai! F Waterpoort trough Sandstone. sntstone. Shale. ,'Tludstone. basait G Thipise trough Sandstone. sBtstone. snale, !Tludstone, basalt H Mesisi strip Calcareous sandstone. congiomerate n Limpopo Highland A Waterberg Sandstone. shale. congiomerate. lava C Pletersburg Plateau GranitIc and crystalline metamorphic rocks ID 3ushve!d BaSin A Northwestern 9ankev-eld Dolomi!e, quartzite, shaie, andesite, diabase B Marico 8ushveld Shaie, quartz:!e and maf!c :nstrusi'les C Sou!hern eankev~ld Shale, quartzite, andeslte, diabase D Katst belt Dolomite, chert and intrvsives, sandstone, shale E VVes:ertl Sushveld Mafie :"'obe Norlte. [magnetite} gabbeo. anorthoslte. syenite. foyeite F Wester" 8ushveld Felsic ~coe Granophyro", granite, dolomite, quartzite. shale. t!""schyte G Springbok Flats Shale. sandstone, mudstone. siits!one, basalt. doler:te H Northern 8ustlVeld Complex Norite. gabbro, granite, rhyoli!e / felslte Shale, sandston~, conglomerate, '/olcsnics ) ~i~de~bEur~U;~';~~d Felslc Lobe Granophyre. granite, volcanics, quart:i!e, sna!e, dolo:nite L Eastern / Northeastern 3ankeveld Dolomite. quartzite, shale, anaesito. diabsse IV Wltwatersrsnd Sasln A Halfway House Dome Gneiss. migmatl!e. granodlorite Regional boundary Subregion boundary CHAPTER G3 SOUTH AFRICAN GROUNDW ATER STUDY G3.1 INTRODUCTION Two aspects are addressed in this study: • What significant groundwater resources, if any, are present directly south of the South African - Botswana border which could make a meaningful contribution to the water resources along the Limpopo River down to the Shashe confluence? • What is the extent of water-bearing alluvium along the Limpopo River? To what extent is it being exploited? Despite the inevitable uncertainty attached to an estimation of this type, it is of interest to note that the quantity of groundwater that could be abstracted from the main drainage region A (the Limpopo Basin in RSA) under foreseeable limitations of economics and practicability has been estimated at 500 m3lff'/yr (DWA, 1986). The demand in 1980 on the groundwater resources was assessed at 361 m3106/yr. The projection for 1990 was 382 m3106/yr. Corresponding figures for the upper part of the Limpopo basin (i.e. omitting secondary drainage regions A700, A800 and A900) are 405, 280 and 296 m3 106/yr. The scope for increased utilisation of groundwater in the upper Limpopo catchment therefore appears to be limited. Apart from bulk supplies that could be obtained from certain sections of the dolomite along the southern edge of the Limpopo drainage region, and excluding alluvial aquifers already exploited for irrigation, groundwater resources elsewhere are such as to be of importance as local sources of supply only i.e. for rural and smaller urban communities. The principle use of groundwater in this drainage region is irrigation. It should be noted that a proportion of the estimated 1980 consumption was undoubtedly derived from surface flow that was directed to the subsurface by the dewatering of alluvial deposits along rivers. This review is restricted to a strip of country about 60 km wide. bordering on the Marico and Limpopo Rivers. and stretching from Derdepoort in the west to the confluence of the Shashe in the east. The strip is divided into the following hydrogeological entities (see Figure G3.1): (i) Hard-rock units Area (krn~ • Crocodile-Marico lowland 5200 • Matlabas catchment 2500 • Waterberg coal basin 3800 • Monte Christo-Alldays belt 9400 • Limpopo Karoo Basin 1000 • Waterpoort Karoo Trough (western section) 1100 G-15 (ii) Alluvial aquifers beneath and along the following rivers: • Marico • Crocodile • Matlabas • Mokolo • Lephalala • Mogalakwena • Limpopo Apart from some notes on the top six rivers, attention will be focused on the Limpopo. A unified description of the alluvial deposits on both sides of the Limpopo river is attempted with the sparse data available. G3.2 HARD-ROCK FORMATIONS G3.2.1 Crocodile-Marico Lowland The lowland adjoins the border between Derdepoort in the west and a point about 40 km south-west of the Buffelsdrift border gate in the east. It is a bush-clad, practically featureless level expanse. Except for the Elandslaagte, the Lengope 10 Kgamanyane and Lenkwane spruits, surface drainage to the Marico and Crocodile Rivers is poorly defmed. Because of a nearly ubiquitous thick soil cover, rocks are poorly exposed. They are comprised of Swazian (Archean) granulite, schist, quartzite,. arkose, banded ironstone, amphibolite, serpentinite and intrusive granite and granite-gneiss. The Swazian basement complex is skirted by a narrow band of Proterozoic intrusive and extrusive rocks, and sediments older than the Transvaal Sequence. In an attempt to improve on the poor results of drilling for water, a considerable amount of geophysical borehole siting was undertaken by the RSA Geological Survey in the period 1952-57. The results of these groundwater investigations were summarized by F W Schumann. Table G3.1 shows the results of an analysis of previously drilled boreholes for which data is available on the data bank of DWA(RSA). Table G3.1 Analysis of Boreholes in the Crocodile-Marico Lowland Rock type Number Percentage Yield Median Percentage of successful boreholes of failures range of yield yielding holes (yield less successful than 0.1 l/s) boreholes 0.1 - 0.9 lIs 1.0 - 4.9 lIs over 5.0 lIs (l/s) (l/s) Swazian metamorphics 549 68 0.1 - 4.4 0.65 60.0 40.0 0.0 G r ani te/granite-gneis s 730 62 0.1 - 5.1 0.71 64.1 35.9 0.0 G-16 Although better results were obtained with geophysics, the failure rate still exceeded 50 percent. There is a tendency for successful boreholes to weaken and fail with time. Schumann estimated groundwater consumption in the area as being between 0.9 and 1.4 m31Q6/yr. At that time groundwater was used exclusively for stock watering and domestic purposes on farms. At the time of Schumann's investigations many farming units lacked a water supply, despite drilling attempts. Although limited volumes of water are required for ranching and despite drilling over many decades, lack of adequate supplies has remained a problem up to the present. G3.2.2 MatJabas Catchment The catchment is predominantly comprised of the Aasvrelkop Formation of the Waterberg Group. This sedimentary succession, 300 to 500 m thick, comprises from the base upwards siltstone, sandstone and shale with interbedded sandstone; followed by alternating siltstone and sandstone (feldspathic, gritty, conglomeratic, ferruginous) with whitish fine-grained sandstone at the top (Jansen, 1982). The strata have been intruded, mainly in the south, by irregular bodies and sheets of diabase, granophyric gabbro, and granophyre. Dykes are prevalent in the north. The dominant strike is WNW to NNW. As these rocks lack significant primary porosity and permeability, their water-bearing properties are "determined by the extent to which they have been fractured and disintegrated by weathering and in the case of the intrusives, also on the extent and depth of weathering. The higher yielding boreholes are found in the low-lying areas, and then mainly confmed to dyke contact zones (Simonis, 1988). A statistical analysis of 515 borehole records (DWA data bank, 1991) produced the following results: Proportion of boreholes that were successful (yield over 0.1 lis) 38.4% Proportion of successful boreholes in the following yield ranges: 0.1 to 0.9 lis 55.1% 1.0 to 4.9 lis 38.4% 5.0 to 9.9 lis 5.1%. over 10 lIs 1.5% A similar analysis of yields of successful boreholes reported by Simonis gave the following: Proportion of successful boreholes in th!? following yield ranges: 0.1 to 0.9 lis 39.7% 1.0 to 4.9 lIs 41.6% 5.0 to 9.9 lis 12.0% ~to~~ ~% 15.0 to 19.9 lis 1.4% over 20 lIs (maximum 31.6 lis) 2.4% The median yield is 1.3 lis. The difference between the data bank and Simonis' figures is ascribable to the fact that government drilling is mainly directed at problem areas, and the results are therefore not fully representative. G-17 As the area is one of cattle ranching and game-farming (with some irrigation along the Matlabas River) limited supplies of groundwater estimated at about 0.5 m3 Hf/yr, are used for domestic purposes and stock watering (see Section G3.3.3 for notes on alluvial aquifers along the Matlabas). To what extent groundwater quality conforms to drinking water standards may be gauged from Table G3.2 below, which is based on the results of 247 sampled boreholes. It is evident that the quality is poor, 42% of the sampleS contain between 1 000 and 2 000 mg/l of dissolved solids. Table G3.2 Groundwater Quality in the Matlabas Catchment Item Recommended Maximum Proportion above limit limit permissible limit (mg//) (mg//) Recommended Maximum limit permissable limit Na 100 400 57% 15% Mg 70 100 18% 7% Cl 250 600 32% 15% S04 200 600 20% 10% F 1.0 1.5 77% 10% N03 6 10 13% 10% TDS 500 2000 84% 13% Note: Limits apply to domestic supplies - see Section G5.3 As the poorer quality water is found in the lower-lying areas where borehole yields tend to be higher, utilization of groundwater on a larger scale than for cattle farming e.g. for irrigation and by small urban communities, should they ever be established in this area, appears problematic, unless desalination is resorted to. G3.2.3 .Waterberg Coal Basin The sedimentary successions as r~vealed through exploratory coal drilling conducted by the Geological Survey comprises the following: Maximum thickness Lithostatigraphic unit Basaltic lava (very localised) 120 m Letaba Formation Fine-grained white and reddish sandstone 125 m Clarens Formation Clayey sandstone, mudstone, marl 100 m Red Beds* Sandstone with purple to brown and drab-coloured mudstone 50 m Molteno* Red, chocolate brown and drab-coloured mudstone 90m Beaufort* Intercalated carbonaceous shale bands and seams of bright coal 85 m Upper Ecca* Sandstone and grit intercalated with carbonaceous shale and seams of dull coal 82 m Middle Ecca* Shale, sandstone and grit 165 m Lower Ecca* * SAGS approved Formation names not available. - . G-18 It should be note that the Clarens Fonnation is equivalent to the Ntane Sandstone in Botswana. Except for some dykes mentioned by Cillie (1951 and 1957) and Cillie and Visser (1945), dolerite intrusions have not been reported, neither have they been encountered in the Groote Geluk open cast mine near Ellisras. The Karoo beds are cut by a number of E-W striking faults. The more important of these are known as the Zoetfontein and Eensaamheid faults. A proper hydro geological investigation of this area has as yet not been undertaken. According to Visser (1952), water is struck in Karoo strata practically anywhere, and at all sorts of depths. The drilling success rate (yields over 0.1 lIs) js 62%. Yields are, however, small to medium. The so-called Red Beds are the poorest aquifers. According to Hodgson (1991), Middle and Lower Ecca sediments at Matimba Power Station near Ellisras yield little or no water. During a visit to Groote Geluk mine, Iscor's chief geologist reported 26 boreholes drilled in the vicinity of the mine in an attempt to locate an adequate water supply for mine construction. With the exception of one borehole, which probably penetrated a subsidiary of the Daarby fault and yielded about 12 lis, 25 boreholes yielded between 1 to 2 lis. The Daarby fault, the contact between the Letaba basalts and the so called Beaufort beds, has been found to be watertight after packer testing in boreholes. In spite of a number of small faults, the steep hydraulic gradients surrounding the open cast mine are further evidence of the Iow transmissivity of the Middle Ecca strata. In the dry season pumping from the mine amounts to about '1 000 m3/day (12 lIs); the mined-out area measures roughly 2 km by 3 km. This volume is composed of two parts: Ca) groundwater flow towards the mine, which is recharged by rainfall and (b) groundwater held in the excavated rock. Without additional data, the relative proportions can not be detennined. A major groundwater resource is, however, not indicated. A statistical analysis of records of 134 holes drilled on farms in the Waterberg Coal Basin yields the following: Proportion of boreholes that were successful (yield over 0.1 lis) 61% Proportion of successful boreholes in the following yield ranges: .0.1 to 0.9 lis 74% 1.0 to 4.9 lIs 22% 5.0 to 9.9 lIs 4% Drilling is comparatively deep with depths ranging from 40 m to more than 100 m, whilst water levels lie mostly between 10 to 40 m below the surface. It should be noted that there are no subdivisions to give infonnation relevant to each particular fonnation. Although drilling results could no doubt be improved by scientific siting of holes, groundwater conditions appear to be much poorer in the Karoo strata on the South African side than across the Limpopo in Botswana. Consumption of groundwater for cattle farming in the Waterberg Coal Basin area is estimated at about 0.7 m3 1{f/yr. This figure excludes groundwater abstracted from the Mokolo River for irrigation. Ellisras, the Matimba Power Station, and the Groote Geluk Coal mine are supplied from Hans Strijdom Dam, whilst irrigation takes place from open water and sand in the Mokolo river (see Section G3.3.4). G-19 G3.2.4 Monte Christo - AlIdays Belt This area is part of the Limpopo metamorphic and granite complex which comprises: • meta-quartzite, magnetite-quartzite, felsite, marble and calc-silicate rocks. • basic and ultrabasic igneous rocks such as amphibolite, gabbro, pyroxenite, anorthosite and serpentine rocks. • granite and various types of para- and orthogneisses. The structure of the complex is one of intensely compressed folds. Aquifers exist in these crystalline basement rocks only where: • they have been converted by weathering processes into a fractured and porous mass which extends to below the groundwater level and/or • open water-fIlled fractures are present in the fresh rock. The area as a whole has not been covered by groundwater investigations. Intensive studies have, however, been carried out west of Beauty, between the Lephalala and Mokolo rivers, and between Swartwater and the Mogalakwena. These two areas are considered fairly representative of most of the Monte Christo - Alldays belt. In these two areas the base of the weathered zones lies prevalently above the groundwater level regardless of its actual thickness. Areas underlain by water-bearing weathered rock accordingly constitute a (very) small fraction of the total. The volume of groundwater in storage is, therefore, very restricted. Furthermore, transmissivities are low and usually discontinuous. This is borne out by the high percentage of drilling failures, low borehole yields and the periodic weakening or drying up of boreholes during extended dry periods. Along certain water divides in the Swartwater-Mogalakwena area, and in areas between the Lephalala and Mokblo Rivers, and west of the latter, there appears to be little hope of providing for the needs of stock watering and rural households from groundwater. It should, however, be noted that conditions are variable. Between the Lephalala River and Tomburke, water supplies are ftrm and more than adequate for ranching. In the neighbourhoods of Tomburke, Mamitz, Baltimore and Tolwe, a total of about 1 000 ha are being irrigated from high-yielding boreholes (Burger, 1991 Pers Corn). There have, however, been claims that ground water supplies in the Tolwe area have been dwindling. This has yet to be proven or disproven. Depending on the irrigation duty required,S to 10 m3106/yr could be abstracted in the four areas mentioned. As far as is known, no hydrogeological study has been undertaken in the areas where irrigation is practised. The Monte Christo - Alldays belt is basically a cattle farming area. Based on a carrying capacity of 15.5 ha per head of cattle and 350 head per economic farming unit, the water requirements, including domestic use are estimated at about 1.75 m3106/yr. It seems unl~ely that significantly more groundwater could be abstracted than is presently the case. G-20 A statistical analysis of 2 733 borehole records yields the following: Proportion of boreholes that were successful (yield over 0.1 lis) 37.4% Median yield less than 1.0 lis Proportion of successful boreholes in the following yield ranges: 0.1 to 0.9 lis 53.2% 1.0 to 4.9 lis 38.5% 5.0 to 9.9 lis 5.5% over 10 lis 2.8% The extent to which groundwater quality in the Swartwater and Beauty areas conforms to the drinking water code is evident from the following table, which is based on an assessment of 211 sampled boreholes. Table G3.3 Groundwater Quality in the Swartwater and Beauty Areas Item Recommended Maximum Proportion above limit limit permissible limit (mg/I) (mg/I) Recommended Maximum limit permissable limit Na 100 400 57% 9% Mg 70 100 27% l7% Cl 250 600 29% 12% S04 200 600 17% 5% F 1.0 1.5 64% 37% N03 6 10 72% 54% TDS 500 2000 98% 27% Note: Limits apply to domestic supplies - see Section G5.3 G3.2.5 Limpopo Karoo Basin The greater part of the area occupied by the basin is taken up by Clarens sandstone with scattered small outcrops of Letaba basalt (see Subregion lE on Figure G3.1). Along the Limpopo River a strip of alluvium up to 4 km wide is present (see Section G3.4.6). The Clarens sandstone is encircled in turn by bands of Bosbokpoort mudstone, shale, mudstone and sandstone (probably of Beaufort age) as well as Ecca sandstone, shale and coal. According to van Eeden (1961), the very fine-grained clayey and sandy sediments (Bosbok.'Poort and Clarens (7)) yield very little water. However, boreholes drilled in zones of indurated sediments along east-west striking dolerite dykes yielded initially up to 3.8 lis. However, the high yields decrease very soon after extraction commences. The results of a statistical analysis of 111 boreholes drilled in the Karoo sediments and scattered throughout the area, are given overpage. G-21 Proportion of boreholes that were successful (yield over 0.1 lIs) 42% Proportion of successful boreholes in the following yield ranges: 0.1 to 0.9 lIs 72% 1.0 to 4.9 lIs 28% It should be noted that the information has not been subdivided to provide information on each formation in the basin. Excluding water abstracted from alluvial deposits along the Limpopo River. demand and consumption for stock watering and domestic purposes is estimated at 0.2 m3 Hflyr. G3.2.6 Western Part of Waterpoort Karoo Trough This area. which lies south of Alldays (see Subregion IF on Figure G3.1), was examined W R G Orpen et al (1982), to determine the availability of groundwater for urban supply. Although Clarens sandstone and older Karoo sediments outcrop in places. the greater part of the trough south of the Vetfontein fault is occupied by Letaba basalt. Aeromagnetic data shows that the area is heavily traversed by east-west striking anomalies indicative of subsidiary faults and/or dykes, which may prove favourable structures for bore holes. Information from 152 boreholes located on 52 cadastral farms south of the Vetfontein fault reveals the following: Proportion of boreholes in the following yield ranges: o to 4.9 lIs 69% 5.0 to 9.9 lis 11% 10 to 19.9 lis 16% over 20 lIs 4% The comparatively large proportion of high-yielding bore holes, coupled with the fact that 79 ha on 8 farms are irrigated with an estimated 0.4 m3IQ6, has led to the recommendation of supplying up to 1 000 m3/day from this area for the proposed expansion of Alldays, and up to 500 m3/day for the adjacent Ganspan township. Groundwater in the basalt has a typical electrical conductivity in the range of 75 to 150 mS/m (500 to 1 000 mg/I) indicating that the water should be suitable for domestic use. The demand for stock farming is estimated at 0.2 m3106/yr. G3.2.7 Concluding Remarks No attempt has been made in this review to provide estimates of volumes of groundwater held in storage in the different hard-rock units, or of its recharge by rainfall. The reason is explained as follows: • A high degree of uncertainty is attached to such estimates. G-22 • Such estimates are not a measure of the volumes of groundwater that can be developed. • As groundwater is a distributed resource, it has to be concentrated in order to gain greater relevance than merely that of a local supply. • Hydrogeological, practical, and economic factors determine to what extent ground water can be salvaged from being lost through natural processes of discharge. • The prevalent low yield of boreholes militates against the use of groundwater for any purpose but local. This means that the utilisation of groundwater in the hard-rock formations in this area is limited to rural domestic and stock watering purposes, and where locally more favourable groundwater conditions pertain, to smaller irrigation projects and lesser urban developments, e.g. Alldays. G3.3 ALLUVIAL AQUIFERS ALONG TRIBUTARIES Although the main emphasis of this study has been on the Limpopo River itself, brief notes have been included on the alluvial deposits along the South African tributaries of the Limpopo, in as much detail as available information allows. G3.3.1 Marico River A strip of alluvium averaging about 700 m wide on the right bank of the river is shown on the 1:250 000 geological sheet (2426 Thabazimbi) from Derdepoort down to the Limpopo confluence. Alluvium has also been mapped along the ElandsJaagte, Lengope 10 Kgamanyane and Lenkwane spruits. At the junctions of the latter two with the Marico river, the alluvial deposits fan out to form delta-like deposits. Neither the thickness of the alluvium nor the depth of the watertable along the Marico is knOwn. G3.3.2 Crocodile River Detailed investigations of the Crocodile River between Koedoeskop and its confluence with the Marico have been undertaken (SS&O, 1990). Two large alluvial aquifers, with an estimated storage volume of 143 m31Q6, lie beneath this 150 km long section of the river. Annual borehole abstractions amount to 60 m3IQ6, for irrigation use. In dry years the volume abstracted exceeds the flow in the river; consequently, when the river does flow there can be a significant reduction in the volume reaching the Limpopo, as a result of recharge to the aquifers. Average recharge from the river to the aquifer was estimated to have been 42 m31~/yr during the period 1982 to 1988. It has been suspected that there may be significant flow from the alluvial deposits into underlying heavily faulted dolomitic formations, but there is insufficient data to be able to prove this. G-23 G3.3.3 Matlabas River Alluvial deposits consisting of sand and silt are mainly conftned to the river bed. South of the farm Colchester 17KQ (60 km upstream of the Limpopo confluence), the sand is too fme-grained for the installation of sandpoints. Simonis (1988), found 20 sandpoints along the stretch downstream of Colchester 17KQ. Yields range from 1 to 22 lis, sand thicknesses from 3 to 10 m. Owing to the damming and abstraction of surface flow, for irrigation in the upper parts of the river, steady supplies are not obtainable from the sandpoints. For this reason little, if any, irrigation is apparently practised from sandpoints. Water from sandpoints is used for drinking and stock watering, being of excellent quality, According to Simonis (1988) boreholes in the hard-rock formations, near the river, yield saline water. However, this could not be verifted by the data analysed for the current study, as reported in Table G3.2, as the information giving the distance of each borehole from the river was not readily available. G3.3.4 Mokolo River According to Mulder (1971), a volume of 85 m3106 of sand underlies the bed of the Mokolo River from the farm Tamboetiekloof 607LQ (about 12 km downstream ofHans Strijdom Dam) to the confluence with the Limpopo, a distance of about 105 km. The thickness of the alluvium decreases downstream from an average of 9 m in the upper part, to an average of about 4 m over the lower 15 km. The maximum thickness probed along sections across the river was 18 m. The volume of water-bearing alluvium flanking the upper stretch of the river between Tamboetiekloof 607LQ and Vogelstruisfontein 472LQ has been estimated at 63 m3IQ6. Because of its clayey composition, alluvium flanking the river channel downstream of Vogelstruisfontein is not thought to contribute signiftcantly to the available groundwater storage. The total volume of potentially water-bearing sand is therefore estimated at 147 m3106• Mulder (197l), puts the porosity of the river sand at 37% and its speciftc yield at 25%. except for the lower 15 km. Owing to the presence of clay and silt layers, a speciftc yield ftgure of 12% is assigned to the alluvium of this stretch. Vipond (1988) puts the speciftc yield of the channel sands at between 20% and 25% and the speciftc retention between 5% and 10%. The volume of groundwater held in the alluvial deposits, when saturated to the river bed surface, is accordingly estimated at about 35 m3106• Rech (1970). conducted a survey of the Mokolo River in November - December 1969. Water was at that time abstracted for irrigation on 42 farms downstream of the, then non-existent, Rans Strijdom dam. Upstream of Wonder Boomshoek 550LQ open river water was pumped on six farms, of which ftve also abstracted water from the river bed. Pumping capacity from the sand and open water totalled 2.36 and 0.35 m3/s respectively. The area irrigated amounted to 2 670 ha The volume abstracted from the sand has been estimated at between 7.5 and 10 m3 106/yr, and from open water at 1.5 to 4.0 ml IQ6/yr. The total amounted to nearly 12 m3 1cf'/yr. White Paper N-70 reports that about 2 480 ha downstream of the dam site was somewhat erratically irrigated by an estimated 15.6 m3106/yr, derived from both the river and its sand bed. After completion of the Rans Strijdom dam, abstraction from the river bed declined. According to Vipond (1988), major irrigators obtain supplies by pumping directly from open water. Major irrigators have a pumping G-24 capacity of about 0.6 m3/s at their disposal. In spite of the problems associated with the operation of sandpoints i.e. breaking of suction and clogging of screens, which must have been at least partly responsible for abandonment of the systems which were existent in 1969, abstraction by wellpoints has been demonstrated to be feasible provided sites are carefully selected, mobilization of clay layers during installation by jetting is obviated, and excessively high pumping rates are avoided. Reinstatement of wellpoints or other sand abstraction systems would lead to a more efficient and continuous utilization of the limited water resources. G3.3.5 Lephalala River The only accessible information is that contained in Rech's report referred to under the Mokolo River. In 1969 an area of 2 499 ha w~ irrigated from wells and sand points in alluvium, from open water, and weirs along a 115 km stretch upstream of the Limpopo confluence. Pumping capacity from the alluvium totalled nearly 1.5 m3/s, and from open water and weirs 1.1 m3/s. The total volume pumped per year was estimated at 12 m3106• G3.3.6 Mogalakwena River The stretch of the Lower Mogalakwena river that was investigated as a possible source of supply to Alldays (Orpen et al, 1982) extends from Lappidood 275MR (on the Alldays - Maasstroom road) to the confluence with the Limpopo River. Although riverine deposits occur throughout this stretch, only seven patches were identified as significant aquifers. It was estimated that 2 m3106 of water is stored in the alluvium and the underlying weathered rock. The annual abstraction was estimated at 0.12 m31~, of which 45% was used for irrigation, the rest supplying domestic and stock watering needs. Releases from Glen Alpine dam provide the bulk of the irrigation requirements. G3.4 ALLUVIAL DEPOSITS ALONG THE LIMPOPO RIVER G3.4.1 Introduction The areal distribution of alluvial deposits along the Limpopo River is shown on overlays of the 1:50 000 map series provided by the RSA Department of Geological Survey. 'The compiler Brandl (1991), provided the following accompanying notes: ItAlluvial deposits are wide-spread along the course of the Limpopo River and they attain a width of about 500 - 1000 m between Cumberland in the south (sheet 2326 DD) and the confluence of the Motloutse and Limpopo Rivers. Only at a few places, where the Limpopo River encounters resistant Archaean gneisses, narrow channels are formed exposing only bedrock. The alluvial fill which is generally 5 - 6 m thick, but probably does not exceed 10 m, is made up of G-25 dark brown silt with interlayered sand and gravel. In places calcrete bands can be present. Remnants of high-level terrace gravel are spasmodically developed. They can occur several kilometres from, and up to 20 m above, the present valley bottom. They probably represent relics of a fonner erosion surface, caused by late Tertiary tectonic activity. Just south of the junction with the Motloutse river, the Limpopo meanders through and up to a 5 km wide flood plain which is more than 20 m thick. It might be partly made up of semi-consolidated Tertiary sediments. Upstream of the junction with the Shashe River, where the Limpopo cuts across fairly soft Karoo rocks, a flood plain is fonned with a maximum width of about 4 km. The alluvial fill which is estimated to be in excess of 10 m, is less silty and more sandy than upstream of the junction with the Motloutse River. Within the flood plain ancient river loops can be identified which are now partly infilled". G3.4.2 Sources of Information As no thorough hydro geological investigation has as yet been undertaken of the alluvium on the South African side of the Limpopo, extensive use was made of existing reports, and in some cases, personal communications and reconnaissance. G3.4.3 Section 1: Chainage 0 to 290 km This section of the Limpopo stretches from the Marico-Crocodile confluence to about 7 km downstream of the Lephalala confluence, and is characterised by: • a very Iow gradient (approximately 1:3 500) • a greater width of alluvium than further downstream. From Buffelsdrift gate to Stockpoort (parrs Halt) the width varies mainly between 1 000 to 2 500 m. Further downstream to the LephaIala confluence the strip is between 500 and 1 500 m wide. • large meanders as well as abandoned meanders on the farms Charlestown 115LQ, Shortlands 117LQ, and on Hartebeesfontein 69LQ (sheet 2327 BC Limpopodraai) as well as on the Botswana side opposite Masseilles 7LQ (sheet 2326 DD Cumberland). • smaller scale meandering as is evident from relics of older channels and vlei areas within the strip of alluvium (sheets 2327 CA Hardekraaltjie and 2326 DB Buffelsdrifhek). • more extensive deposits at the junctions of the Matlabas, Bonwapitse, Mahalapswe and Lephalala Rivers. Vertical movement in Tertiary times along the east-west trending Constantia, Mahalapswe, Zoetfontein (and subsidiary), Hardekraaltjie, Eensaamheid and BOleleme faults appear to be the cause of the low river gradient, the consequent meandering and deposition of alluvium. G-26 Section la: Upper Stretch to MahaJapswe Confluence (Ch 0 to 160 km): Hobbs and Esterhuyse (1983), carried out a hydrogeoJogical survey of the Limpopo between the Crocodile and MatIabas confluences. On Cumberland 9LP two boreholes, more than 1 000 m from the river bank, have penetrated sand to depths of 15 and 20 m below the watertable, which presumably does not differ greatly from river bed level. On Olifantshoek lLP where the Matlabas joins the Limpopo, and approximately midway between the Limpopo and MatIabas and about 1 000 m distant from either, a number of boreholes have shown a thickness of 8 - 11 m of water-bearing alluvium (see Drawing No MSC 1/115) Assuming a specific yield of 0.1, Hobbs and Esterhuyse (1983), estimated that a volume of 23 m3 l(f of ground water is stored in patches of alluvium along the right-hand bank between the Crocodile and the MaUabas. They consider that 17 m31Q6 is extractable (and renewable by floods). Storage on the Botswana side could be of the same order. • On Twee Rivier 279LQ, immediately east of the MatIabas confluence, a bore hole one or two kilometres north of Mr J N Beukes' farmstead penetrated unconsolidated material and a pebble bed to a depth of 31 m (as interpreted from the drillers' log and boring residues at the bore hole site). The hole is situated on a rise about midway between the eastern and western limbs of a large river meander. With the watertable at 22 m below the surface, a thickness of alluvial deposits of 9 m below river bed level is indicated (the Eensaarnheid fault crosses the Limpopo in this vicinity). The following paragraph from Section G2.4.9 is reiterated here for clarity: At the confluence of the Mahalapswe and Limpopo (Limpopo farm) a faulted block of Karoo strata is overlain by gravels, coarse sands and superficial silty sand and marls. The aquifer is assumed to have a thickness of some 15 to 20 m and with an assumed storage of 20%, a total recoverable (meaning "mineable" without regard to recharge) resource of 30 m3 IQ6 (in Botswana) and an inflow of 0.6 m31(f/yr from the Mahalapswe river area has been calculated by De Vries (1985). Aquifer transmissivity is very high at 1 500 m2/d. On the hydrological reconnaissance map sheet 8 of the Republic of Botswana, the estimated production potential at Limpopo Farm is indicated as 5 000 m3/d (60 lis). The areal extent of the aquifer as shown on the map appears to be about 15 km2• The safe or fmn yield of this aquifer obviously depends on "'the frequency and infIltration volumes from floods in the Mahalapswe and Limpopo Rivers. Viewed superficially similar favourable ground water conditions may exist at the confluence of the Bonwapitse and the Limpopo on the Botswana side, and on the farms Boompan 239LQ, Witkopje 238LQ, Doornkopje 235LQ and Koert Louw Zyn Pan 234LQ, bordering the river on the South African side (see Album Drawing MSC 1/115): a subsidiary of the Zoetfontein fault crosses the river on the next farm downstream. Considering the available evidence it does riot seem unreasonable to speculate that a considerable volume of extractable groundwater, perhaps 100 m3106, or even more, could be present between the MatIabas and Mahalapswe junctions. The nature of the alluvial deposits will determine the yields of boreholes. G-27 Section 1b: Mahalapswe to LephaJaIa (Ch 160 to 290 km): Although conditions downstream of the Mahalapswe junction appear to be less favourable for the occurrence of similar volumes of water-bearing alluvium, the stretch to Grootwater 29LQ (past the Lephalala confluence) should not be discarded outright as being of little or no ground water potential. Excepting the frrst part down to Sannandale 9LQ (sheet 2327 AD Stockpoort) with many outcrops in the river bed, few outcrops are shown on sheets 2327 BC Limpopodraai, 2327 BA Happy-go-lucky and 2327 BB Tomburke. There is also evidence for, or indications of, the existence of older channels deeper than the present river bed. In the cut-off meander on Charlestown 115LQ and Shortlands 117LQ (sheet 2327 BC Limpopodraai) between 18 and 24 m of presumably alluvial material was penetrated in boreholes before encountering rock (Pretorius, 1991). This is corroborated by the log of a borehole drilled on the farm in 1982 (position undetennined) in which 22.6 m of soil: boulders and sand was penetrated before striking bedrock. With a water level at a depth of 11 m, an older channel at this depth below the present river bed is indicated. According to Duvenage (1991) alluvium extends some 15 to 20 m below river bed level directly east of the Mokolo junction, as indicated by electrical resistivity depth probing. On hydrogeological reconnaissance sheet 8 of Botswana, good groundwater development prospects are shown at Sunnyside farm (about midway between the Mokolo and Lephalala confluences). This farm lies between two horseshoe bends in the river on Villa 40LQ and First Hope 37LQ (sheet 2327 BB Tomburke). Relatively extensive alluvial deposits on these farms are shown on the map and also on the Botswana side. An alluvium "bulge" is present opposite the Lephalala junction. There is some uncertainty about the quality of groundwater contained in infilled older channels away from the present river course. Hobbs and Esterhuyse (1983), ascribe the saline water found in some holes in the section investigated by them, to those holes having struck poorer quality water in the underlying bedrock. Water in the cut-off meander on Charlestown 115LQ is also saline. Summary In conclusion, the stretch of river between the Crocodile and the Lephalala has noteworthy abstractions of groundwater on the South African side on: • Buffelsdrift 3LP (one borehole yields 31 lis) • Olifantshoek lLP (two boreholes, yielding 31.5 and 1261/s). Groundwater is used supplementary to river water (DWA, 1989a). According to Hobbs and Esterhuyse (1983) 42 ha on Olifantshoek lLP was irrigated with groundwater . • Charlestown 11SLQ 35 ha maize (DW A 1989a). The extent of groundwater utilization on the Botswana side is not known. Granting that older and deeper infIlled channels exist, the question arises as to why these have not been discovere~ and exploited by more farmers. Apart from possible agricultural economic and other reasons, older G-28 infIlled channels can be located and traced satisfactorily only by means of geophysical surveys supported by exploratory drilling. Fwihennore, the development of high-yielding boreholes in alluvial deposits require expertise which is not generally available. The potential for larger scale groundwater development for urban supply particularly between the Matlabas and Mahalapswe junctions requires investigation. Artificial recharge may also prove a worthwhile proposition. G3.4.4 Section 2: Cbainage 290 to 450 km This section of river stretches from just downstream of the Lephalala to about 20 km upstream of the Motloutse, and is characterised by: • a steepening of the gradient from about 1:1 800 to 1:75. • a narrowing of the alluvial strip to between 300 to 600 m with local widening up to 1 000 m. A meander on Klippan 25LQ and Eersteling 138MR (sheet 2227 DD and 2228 CC Swartwater) has an older cut off channel. On these two farms, and on the adjoining Doornplaats 26LQ and Welvaart 27LQ, separate areas of alluvium are present indicating older courses of the Limpopo. They appear to be of little or no groundwater importance. On Klippan and Sterkloop borehole water supplements surface supplies for irrigation. On hydrogeological reconnaissance map 8 of Botswana, Seleka Ranch is indicated as having good ground water development prospects. Seleka Ranch lies to the east of the meander described above and directly opposite Eersteling 138MR and Sterkloop 137MR. The groundwater on Seleka Ranch is found in fractures parallel to the amphibolite intrusions and is apparently not replenished from the Limpopo. Few outcrops occur over the 28 km stretch from Tulbagh 135MR to Sekombo 68MR (sheets 2227 DD, 2228 CC Swartwater and 2228 CA Mabalel). Farms that have, however, been identified during a reconnaissance survey (Bush, 1989) as having reasonable prospects for the installation of wellpoints are Selous 60MR, just below the confluence of the Lotsane, Tuli 66MR, Wederdooper 55MR, Du Plessis 68MR, and Umzumbi 21MR (sheets 2228 CA Mabalel and 2228 CB Maasstroom). Nine kilometres downstream, reasonable prospects appear to exist on Matikule 23MR and Bambata 33MR. Few outcrops occur in the river bed between Umvoti 167MR and Kobeenpan 17MR (sheets 2228 DA Usutu, 2228 DB Gregory and 2228 BD Platjan). The following farms in this 30 km stretch, which is characterised by a flattening of the river gradient, Marlow 184MR, Moreland 182MR, Mayholme 196MR and Kobeenpan 17MR, are said to offer reasonable possibilities for the installation of wellpoints. The only abstraction, however, is on Platjan 198MR at a rate of 1.3 m3106/yr (DWA, 1987). According to Orpen et al (1982) no potential for profitable large scale irrigation from groundwater exists between Shangai 9MR and Kruidfontein 1MR, and upstream on Eerstekrans 16MR and Bievack 14MR. In summary, limited prospects for abstraction by means of wellpoints exist on a number of farms along the 175 km stretch between Grootwater 29LQ and Kruidfontein IMR. Apart from these possibilities, the most promising area for ground water abstraction seems to be the meander on Klippan 25LQ and Eersteling 138MR. G-29 Owing to insufficient data, current groundwater abstraction on these two farms is not quantifiable. It probably does not exceed 1 m3106/yr. This means that the total consumption of alluvial groundwater along the South African side of this section of the Limpopo probably does not exceed 2.5 m3106/yr. G3.4.5 Section 3: Chainage 450 to 470 km This section stretches to just downstream of the Motloutse confluence. At the junction of the Motloutse and Limpopo, alluvium covers an area of approximately 35 km2 above Poortjiesberg (alluvial deposits along the Motloutse River upstream of the so-called Solomon's Wall are not included). The alluvial aquifer is stated to be 5 to 20 m thick on the Botswana side (see Section G2.4.9). Orpen et al (1982) described an alluvial section of about 30 m thick, consisting of two layers of sand, with intercalated clay and silt lenses, separated from each other by clay and silt layers about 14 m thick. Both sand beds are water-bearing. The depth to the watertable is not mentioned. Insufficient data is available to estimate the volume of ground water held in storage. According to Orpen et al (1982) an area of 184 ha was irrigated from wellpoints in 1982; the 1988-89 Water Affairs survey (DWA, 1989a) seems to be incomplete. The quantity abstracted was estimated at 1.8 m3106/yr. Abstraction on Talana Farm is estimated at about 2 m3106/yr (see Section G2.4.9). It is, however, not clear how much water is abstracted below the Solomons Wall i.e. from Limpopo alluvium. G3.4.6 Section 4: Cbainage 470 to 510 km This section extends between the confluences of the Motloutse and Shashe rivers. Alluvium occupies an area of about 100 km2 between Poortjiesberg and the Shashe confluence. The width of the alluvial strip varies roughly between 500 and 1 000 m for the first 8 km. From here onwards the width expands to about 4 km and more. According to Orpen et al (1982), the riverine deposits attain thicknesses of 24 to 35 m. However, at Ponts Drift the deposit becomes relatively thin owing to bedrock ridges whiC;h cut across the river. Orpen et al (1982), state that 370 ha were under irrigation from boreholes on Koedoeskop (Hills tone) up to, and including, Ponts Drift The 1988-89 survey of Water Affairs (DWA, 1989a) did not cover Koedoeskop 8MS and Parma 40MS. In 1982 the area irrigated downstream of Ponts Drift to the Shashe confluence totalled 881 ha which required abstraction of 8.8 m3106/yr, assuming an irrigation duty of 1 000 mm/yr. The irrigated area has expanded considerably since 1982. According to the 1988-89 survey, 1 412 ha were irrigated. It is estimated that about 30% of the irrigation requirements are obtained from the river, when flowing, and from a weir on Den Staat 27MS. The balance is groundwater. On Greefswald 37MS a groundwater supply, pr~sumably of the order of 1 000 m3/d (12 l/s) , is being developed oppOsite the Shashe confluence for the Venetia diamond mine. G-30 No information on abstraction is available for the Botswana side. A satisfactory estimation can not be made of the volume of water stored in the two areas - above and below Ponts Drift. The safe or fIrm yield will be largely determined by the frequency of floods in the Limpopo and tributaries, and the volumes of water infiltrating the river bed. G3.4.7 Seepage from the River Groundwater levels along and perpendicular to the Limpopo are too sparse and widely spaced to gain an idea of the extent of lateral recharge away from the river. Owing to the general low transmissivities of the bounding hard-rock formations, and the tendency for a lateral fIning of the alluvium away from the river course, it would appear that recharge is practically confmed to the alluvium, where it is being depleted by the natural processes of effluent seepage and evapotranspiration, and/or pumping. G3.4.8 Conclusions and Recommendations It is not possible at this stage to assess the volume of water that can be abstracted from the alluvium on a regular basis (and be recharged by the river). Apart from the prolific aquifers in the two lowest river sections, this source is not currently being exploited to any great extent. It is recommended that the section between the Matlabas and Mahalapswe rivers (see Drawing No MSC 1/115) be subjected to a detailed study to quantify its apparent potential as a regional resource. The objectives of such a study would be: • to determine the extent of the aquifer • to determine its storage co-efficient and transmissivity • to determine its recharge characteristics. This would allow a reservoir simulation to be undertaken to determine its long term yield at different reliabilities. The aquifer could also be included in a system analysis to optimise its contribution to a regional scheme. Determining the extent of the aquifer would require extensive geophysical surveys (electromagnetic and electrical resistivity) with calibration by probing and drilling. The storage and recharge characteristics would have to be determined by extensive pump testing, and by monitoring the watertable reaction to river flows. G-31 ) , ..;; G·32 CHAPTER G4 SEDIMENT YIELD ANALYSIS G4.1 INTRODUCTION This analysis was undertaken to establish the likely rates at which reservoirs being contemplated along the Limpopo River would lose storage capacity through sedimentation. Considering the highly variable discharge patterns of the Limpopo River it must be assumed that the sediment loads will be even more variable. It is expected that in order to determine the average sediment load of the river by means of stream sampling, a continuous record of at least 10 years would be required in order to obtain a meaningful value. Sediment sampling on the Limpopo river would be particularly difficult and costly due to the short duration of high flows and the coarseness of the sediment particles. Stream sampling is therefore not recommended. It is common practice in South Africa to derive sediment yield figures from recorded yield values for neighbouring and other comparable catchments as well as from generalised sediment yield maps, with due consideration of conditions within the specific catchment. A revised sediment yield map for South Africa is currently in preparation, and as part of that study, measured sediment values in most large South African dams have been collected. The sedimentation engineer travelled through parts of the catchments during the period 7 to 9 February 1991 in order to familiarize himself with parts of the catchment which he had not seen before, especially on the Botswana side of the river. On the basis of the recorded sediment yield values and field observations it has been possible to derive confident estimates of sediment yields for the dams under consideration. G4.2 RECORDED SEDIMENT YIELD VALUES Sediment yield values which have been recorded within and around the Limpopo catchment are moderate to Iow by southern African standards. Values recorded within the catchment are shown in Table G4.1 and on Drawing MSC 1/116, but values recorded in surrounding areas have also been considered. In Botswana these include Shashe dam (108 tjkm2/yr) and Madabe dam (300 t/km2/yr). In South Africa there are numerous records in surrounding areas. Most of the yields recorded in the catchment are lower than 100 tjkm2/yr. The most significant exceptions, where yields higher than 100 tjkm2/yr have been recorded, are Hartebeespoort and Bon Accord dams. Because the values for these dams are out of line with others in the vicinity, they have been subject to further investigation. It is now believed that the fact that they receive a high proportion of urban run-off and pollution, contributes significantly to the higher rates of observed sedimentation. Apart from the collection of urban waste, pollution has led to hyacinth proliferation on these dams: The dying hyacinths, especially when killed artificially, sink to the bottom and increase the volume of observed sediment deposits. G-33 Table G4.1 Sediment Yields Recorded in Upper Limpopo Basin Subcatchment Dam name Effective Period Yield area (t/km2/yr) (km2) Notwane Notwane 3545 1973 - 86 11 Nnywane 238 1964 - 83 21 Marico Klein-Marico 1 180 1934 - 83 21 Marico Bosveld 1219 1933 - 77 52 Kromellenboog 606 1955 - 83 132 Crocodile Lindleyspoort 705 1938 - 80 83 Koster 280 1964 - 80 36 Olifantsnek 492 1928 - 88 105 Bospoort 588 1953 - 69 76 Buffelspoort 119 1935 - 80 84 Rietvlei 479 1934 - 77 40 Hartebeespoort 3633 1923 - 79 256 Bon Accord 315 1925 - 80 178 Roodeplaat 684 1959 - 80 105 Klipvoor 5005 1970 - 87 27 Bierspruit 1 330 1960 - 80 20 Mokolo Hans Strijdom 4319 1975 - 88 11 Mogalakwena Welgevonden 166 1954 - 77 6 Doomdraai 579 1953 - 79 98 Combrink 174 1964 - 78 15 Glen Alpine 10 713 1967 - 79 10 Even though there are generally few farm dams on the Botswana side, it is considered that the sediment yield from the Botswana side is likely to be lower than from the South African side for the following reasons: flat slopes • stable soils • limited sc~e of tillage • relatively good vegetal cover • few signs of serious erosion. Considering all the information available, it is proposed that the following average yield values be used for planning pUIIJOses: Buffelsdrift 130 t/km2/yr Selika 120 t/km2/yr Ratho 80 t/km2/yr G-34 These yields represent the maximum average yields which can reasonably be foreseen for the large catchments involved, even though they are lower than those indicated by the generalized yield map for Southern Africa. Application of these values with the effective catchment areas of the various dams will provide the maximum yields which can be foreseen at this stage. G4.3 EFFECTIVE CATCHMENT SIZES As a number of storage dams, which trap virtually all sediments from their catchments, exist within the catchments of the proposed dams, the effective sediment yield areas for the proposed dams are significantly smaller than their gross catchment areas. Calculations of the effective catchment sizes are included in Table G4.2. Table G4.2 Effective Catchment Areas Dam site Gross Upstream dams Catchment Effective catchment area . catchment area (km~ area (km~ for dam site (km~ Buffelsdrift 62090 Gaborone/Notwane 3981 Molatedi 8422 Bierspruit 1330 Vaalkop 6110 Roodekoppies 6130 Klipvoor 6138 Bokaa 3570 Total , 35681 26409 Selika 98630 Total upstream of Buffelsdrift 35681 Hans Strijdom 4319 Total 40000 58630 Ratho 150510 Total upstream of Selika 40000 Glen Alpine 11632 Total 51632 98878 G4.4 AVERAGE VOLUME LOSSES With the effective catchment sizes and average yields known, the average rates of volume losses can be determined. As the sediment loads are expected to contain a large proportion of sand, an average constant value of 1 350 kg/m3 has been used in translating tmids into storage losses, with no allowance for consolidation with time. The results are presented in Table G4.3. G-35 Table G4.3 Results of Sediment Yield Analysis Dam site Effective Unit yield Annual volume catchment size (t/km2/yr) loss 2 (1crn ) (l06m3) Buffelsdrift 26409 130 2.54 Selika 58630 120 5.21 Ratho 98878 80 5.86 G4.5 CONCLUSIONS From these calculations it is expected that the storage volume losses after 50 years will be no more than: 127 m3 1(f for Buffelsdrift 260 m3 1(f for Selika 293 m31~ for Ratho Whilst it is believed that these values might prove to be conservatively high, it would be unwise to adopt lower values for design purposes as extreme flood events may lead to average values of this order, even though the catchments are relatively large. In conjunction with the current preparation of a new sediment yield map for Southern Africa, it should be possible to provide probabilistic exceedance values to the sediment yield values for these dams. The results of such calculations could be available towards the end of 1991. G-36 CHAPTER G5 WATER QUALITY SITUATION ASSESSMENT GS.l INTRODUCTION This chapter describes a situation assessment of the water quality of the main river channel and tributaries of the Limpopo River, as well as water quality simulations of the proposed impoundments at Buffelsdrift and Selika. The assessment is carried out to determine the suitability of the surface water resources for a variety of uses. Hence, water quality is defined in terms of "fitness of use" with guideline concentrations given for the following uses: domestic, agricultural (livestock watering, and irrigation), industrial, protection of aquatic life, and recreation. The water quality assessment is undertaken for the tributaries and the main river channel of the Limpopo River using both statistical and deterministic techniques. This information is used in conjunction with mass balance calculations to assess the water qUality in the proposed impoundments located on the main river channel of the Limpopo River. Conclusions are made with regard to the variation in water quality of the river basin. Recommendations are made for additional monitoring and modelling required in Stage II of the study. GS.2 AVAILABILITY OF WATER QUALITY DATA Water quality and river discharge data for sampling stations located in the Republic of South Africa were obtained from the Hydrological Research Institute and the Directorate of Hydrology of the Department of Water Affairs (RSA). Table G5.1 shows information on the data records for each of the sampling stations and Figure G5.1 shows the location of the sampling stations. Water quality data for sampling stations located in Botswana were referenced from Appendix D of BNWMPS, Volume 4 (SNIEC, WLPU and SGAB, 1990). It should be noted that the total number of water samples collected for rivers and reservoirs from the Limpopo basin in Botswana is 48, with only one sample collected at most of the stations. In comparison, the total number of samples collected for rivers and reservoirs in the South African portion of the basin is 1 275, with records extending up to 19 years. Figure G5.2 shows the duration of the data records for the sampling stations in South Africa. GS.3 WATER QUALITY GUIDELINE CONCENTRATIONS The various users of water, namely domestic, industrial, agricultural, recreational and the aquatic environment have different and conflicting water quality requirements. To assess the water quality of a particular river a comparison must be made between the measured water quality and the quality requirements of each water user. Table G5.2 shows the water quality guideline concentrations for six water users, compiled from Kempster et al. (1982), SABS (1984), World Health Organization (1984), Kempster and Smith (1985), and Kempster and van Vliet (1988) for the variables for which data have been obtained. In Section G5.8 recommendations are made with regard to the monitoring of additional water quality variables. G-37 Table GS.l Water Quality Sampling Records for RSA Sampling Stations Station Location Start date End date Number Variables Basin area 2 of (km ) records Al H002 Dinokana 77/07/28 90/06/02 83 Major A2H037 Crocodile River 85/01/24 90/12/27 100 Major 23762 A3H007 Marico River 78/09/05 84/11/05 173 Major 8685 A3H037 Marico River 88/03/11 90/04/04 14 Major & metal A4HOO4 Matlabas River 71/09/28 90/04/11 161 Major 1050 A4H007 Tambotie River 77/12/27 89/12/20 147 Major 398 A4H010 Hans Strijdom Weir 84/01/23 90/05/30 48 Major 4319 A4R001 Hans Strijdom Dam 81/03/11 87/03/30 18 Major 4319 ASH006 Limpopo River 80/01/12 88/02/23 126 Major 98240 ASR002 Susandale Dam 78/11/07 90/06/26 35 Major 3610 A6H009 Mogalak.wena River 71/10/01 90/12/18 273 Major 14733 A6R002 Glen Alpine Dam 75/11/12 80/03/10 47 Major 11 292 A7HOO4 Limpopo River 80/01/10 88/05/17 48 Major 201000 Notes: The term "Major" refers to the availability of major ion analyses (e.g. pH, EC, TDS, Ca, Mg, K etc.). The term "Metals" refers to the availability of trace metal analyses. GS.3.1 Domestic Use The specifications of SABS 241 (1984) and the recommendations of Kempster and Smith (1985), are intended for purified potable water. In order to manage river systems, guideline concentrations for raw water have been developed by Kempster and van Vliet (1988). Table GS.2 shows; the guidelines for domestic use expressed as a 50 percentile "recommended limit" and a 90 percentile "maximum allowable limit". The table also includes recommended standards suggested for Botswana (BNWMPS, Volume 4, Appendix D). These recommendations also include "permissable limits". The figures given in Table GS.2 are for comparative purposes and the original reference should be consulted for further details. It should be noted that the guideline limits should not be applied a~tomatically, they merely serve to provide an initial indication of problem areas. Occasional consumption of water with characteristics inferior to those defmed as "maximum allowable limit" will not necessarily cause harm. Once the details of a source and potential uses are finalised then more detailed consideration can be given to the specific circumstances. GS.3.2 . Agricultural Use In Table GS.2 the agricultural use of water has been divided into livestock watering and irrigation. The guideline G-38 Figure G5.1 Location of Water Quality Sampling Points X\ LEGEND DAM SITES SCALJ: Okm 50 100km F"""'W""'*-'~--.-.. A lH002 EXlSTlNG $AMPLlNG ?C!N"TS o PROPOS€D SAMPUNG ?eMS NUMBER STATION RIVER OF SAMPLES A1H002 Notwane t·:.::*A*AAVN:ii\~Vi«f:u;p»#MyMW~~~~t~Mf:·;;:wt;.~.2~.:.~;~~.~.~.±~~ 83 A2H037 Crocodile ~ite~~~:gw~: 100 A3H007 Marico m~~~ 173 A3H037 Marico ~ 14 A4H004 Matlabas ~t~«m.~{i1w.~tf.~' A4H007 Mokolo ~:·2W~~9·:::'~~·:;:·z.:.w.·~:t:::·X·:·~·~~~~~~~.fiw.·~·r~·?::.:·:;}!;:;:·:·:·:;:::~·}:.w.·:.:R·~{·i?.:!·:.:::·:·~ 14· 7 A4H010 Mokolo ~·:vi::i\·i~~·:\'_:~·::~,:·::.:·:,:.;\·:.;;:.:.:.:\\':.:\.;.:.::.".j:w.¥.::'::'1 4~ 8 t:J w::::;:::jW~:::;:~*:.:t:::::§::::f~:j >== A4R001 Mokolo 18 "'i PJ ...... o· A5H006 Limpopo ~~:-;ti2·fJ:;:ilit>;'\~7.ID:.!.~:::.m':!.~~-»2~:·:·:~r~·:·:·:·:·:·:~·~:~+f·:~·:·:·:·:·:':·:·::::?g 126 ;:l /------/------1------1------1------o H-, A5R002 I Lephalala ;~~~"r§i:3~~~""+:-»»:-;:-:,.;Z-:-~::;:-;:-!-»f,;PB;;:0g:N:~:!-:0:0»:::::0?:s1 3 5 ~ \------PJ ,?~:l2(~§i:N»§:i:0?*:-:-:-:-:-;.:?_l~:-":,;;»t;:::§::;:-;;;:-:-",-:,-:-:-:-:-::~;§:,»f.'0:N:«-:-:-:-:-:-:-:t::::0:,::::;:':;:'~;,@0%:0f.:0:~*::;~::::0:0f.»:;1 2 7 3 ~ A6H009 I Mogalakwena "'i ------1------\------1------1------.. _------1------/ /:) >== A6R002 I Mogalakwena i"-;Ff.w::l'iW'f?i~:'-'''-w.. , .. -w;~:~ 4 7 e:. 1------/------/------/------1 1------/------/ ~. A7H004 Limpopo ~~~:~~*:::0??:!::;:~~0:«j 4-8 t:J ...... PJ PJ 1970 1975 1.980 1985 1990 'Tl ;;d (/';: (]) C o "1 o (1; "'i Cl 0.. VI (/) i·.) concentrations are expressed as a 90 percentile limit. It should be noted that water quality for irrigation use is dependent on the type of crop irrigated and the soil characteristics. Plant species vary considerably in their sensitivity to certain elements, e.g. boron, chloride and lithium. Citrus trees are sensitive to all three of these elements, and for the irrigation of citrus the minimum chloride concentration is generally used. Table GS.2 Water Quality Guidelines for Different Uses Domestic Livestock Irrigation Protection Recreation Proposed watering of for aquatic Botswana life (Domestic) P50 I P90 P90 P90 P90 P90 pH 6-9 5.5-9.5 4.5-9 6-9 6-9 6.8-8.5 EC 70 300 460 80-550 see text 1000 TDS 500 2000 Calcium (Ca) 150 200 1000 1000 Magnesium (Mg) 70 100 500 300 1500 Potassium (K) 200 400 50 Sodium (Na) 100 400 500 200 Total alkalinity 300 650 >20 Chloride (Cl) 250 600 1 500 70-150 400 250 Fluoride (F) 1 1.5 2 1 1.5 1.5 Silicon (Si) 18 50 Sulphate (S04) 200 600 1000 200 1400 400 Ammonium ~) 1 2 1.00 0.8 Nitrate (N03) 6 10 20 10 Phosphate (P04) 0.06 0.10 0.10 Turbidity 5 250 50 5 NOTE: P50 refers to the "50 percentile recommended limit" and P90 refers to the "90 percentile maximum allowable limit". Concentrations are expressed in mg/l (nitrate as mg of Nil), Conductivity (EC) expressed in mS/m, and turbidity expressed in NTU. GS.3.3 Industrial Use The water quality requirements of a particular industry depend on the purpose of the water. For example, the iron and steel industry require water of a lower quality to that required for the food processing industry. As an indication of the water quality requirements for industrial use it can be assumed that if a particular water source complies with the quality guidelines for domestic use then the water will be suited to most industries. G-39 GS.3.4 Protection of Aquatic Life Kempster et al. (1982) review the international water quality criteria relating to the protection of aquatic life. Table GS.2 shows the guideline concentrations for a number of inorganic determinants. The guidelines for electrical conductivity (EC) and temperature depend on the species present as well as on the local conditions. Fish are more sensitive to sudden changes in the electrical conductivity, or temperature, than to the absolute values for these determinants. The toxicity of the metallic elements (lead, zinc etc.) to fish is higher in low calcium content water, or in water with a low conductivity. It should be noted that Table GS.2 does not include organic compounds and trace metals. These are discussed in Section GS.8. GS.3.5 Recreational Use Table GS.2 shows the water quality guideline concentrations for recreational use of surface waters. Macrophytes, algal scums, E.coli, Bilharzia, malaria, and flying insects are additional factors which can detract from the recreational use of a waterbody and at present no information is available for the rivers in the basin. GS.4 WATER QUALITY ASSESSMENT: TRIBUTARIES This section describes the water quality assessment of the ten major tributaries of the Limpopo River. The assessment includes: • Statistical analysis of the data for each tributary to determine the exceedance, or non-exceedance, with the guideline concentrations shown in Table GS.2. The statistical analysis includes calculation of the minimum, SO percentile (PSO), 90 percentile (P90) and maximum values for each variable, see Table G-A.l of Appendix G-A. • Mass balance calculation to show the relative total dissolved salts (TDS) contribution from each tributary to the main river channel, see Table G-A.2. The numerical methods used to calculate the mass balances are described in Section G-A1 of Appendix G-A and are based on a runoff sequence generated for a period of 64 years. • Trend analysis to determine the temporal change in water quality of each tributary. Methods used in trend analysis are described in Section G-A2 of Appendix G-A. GS.4.1 Marico River A study of the surface water quality of the Marico River catchment is given by DWA(RSA) (1989b) and (1990). It is reported that no significant point source discharges influence water quality in the lower reaches of the river. Assessment of the water quality of the Marico River is based on the data for stations A3H007 (now defunct due to the construction of the Molatedi Dam) and station A3H037 at Mooiplaats diversion weir, see Figure GS.I. The combined data set for both stations comprises 187 records which are analyzed statistically and shown in Table G-A.1. Comparison of the 90 percentile values in Table G-A.l with the guideline concentrations in G-40 Table G5.2 show the river water is suitable for domestic, agricultural, industrial and recreational use. Based on major ion composition, the water should have no detrimental influence on aquatic life. Table G-A.2, in Appendix G-A, shows the mean annual runoff and TDS load exported from each major tributary in the basin. The mean annual runoff of the Marico River amounts to 6% of the total tributary runoff. The total dissolved salts (TDS) load exported amounts to 7% of the total tributary export. Consequently, the Marico River has a relatively minor runoff and TDS export potential in comparison with the other tributaries. Trend analysis shows negligible change in the major ion composition or TDS concentration for the period 1978 to 1990. G5.4.2 Crocodile River A detailed study of surface water quality in the catchment was carried out as part of the Crocodile River Catchment Study (SS&O, 1990). Using the electrical conductivity (EC) as a measure of the water quality it was found that the municipal, industrial and agricultural discharges were substantially diluted by runoff from tributaries in the lower catchment. Assessment of the water quality for the most downstream point on the Crocodile River is based on the data for station A2H037, see Figure G5.1. This station is located 70 km upstream of the Crocodile/Limpopo River confluence and has a data set of 100 records beginning in 1985. Table G-A.1 shows a statistical analysis of the data set. Comparison of the 90 percentile values (in Table G-A.1) with the guideline concentrations (in Table G5.2) indicate the water is suitable for domestic, industrial, and recreational use, as well as livestock watering. Due to the exceedance of the 50 percentile value for sodium and chloride, the water may cause problems for the irrigation of certain sensitive crops such as tobacco and citrus. This is confirmed by calculating the sodium adsorption ratio (SAR) which shows the water is classified as Class 2, causing problems for certain crops. From statistical analysis of the major ions the water should have no detrimental influence on aquatic life. From Table G-A.2, the mean annual runoff of the Crocodile River gives rise to 29% of the total tributary runoff and to 38% of the total tributary export of TDS. Consequently, the Crocodile River has a relatively major runoff and TDS export potential in comparison with the other tributaries. Trend analysis for the major ions reveals no change in water quality over the period 1985 to 1990. Should the data record have been longer it would probably show that there has been a considerable increase in sodium and chloride export in the catchment. This trend is due to the increase in effluent discharges and irrigation return-flow to the river over the past 20 to 30 years. Analysis of the ionic composition for station A2H025 on the Crocodile River, 40 km upstream of station A2H037, shows an increase in chloride concentration of about 4% over a ten year period, 1980 to 1990. G5.4.3 Notwane River A water quality monitoring station, A1H002 is located in the upstream section of this river, but the station is too far upstream to be considered representative of the river as a whole. While results from this station are included in Table G-A.1, the data has not been used further in this study. A description of the water quality o~ the Notwane River is given in BNWMPS (SMEC, WLPU, SGAB, 1990 Appendix D). The upper reaches of the Notwane River have observed TDS values of 87 to 136 mgjl. Downstream of Gaborone, the TDS values increases to 536 mg/I due to point source discharge and urban runoff G-41 from the city. An assessment of the water quality for the Notwane River is based on 4 samples presented in BNWMPS. No statistical analyses are performed but individual results are compared with the guideline concentrations in Table G5.2. The comparison reveals the water is suitable for domestic, industrial and recreation use, as well as livestock watering. However, the elevated chloride concentration may cause problems for the irrigation of certain sensitive crops. The water should have no detrimental influence on aquatic life, in terms of the major ions analyzed. From Table G-A.2, the mean annual runoff of the Notwane River gives rise to 3% of the total tributary runoff and to an estimated 9% of the total tributary export of TDS. Consequently, the river has a relatively minor contribution of runoff and moderate export of TDS. Trend analysis was not possible due to lack of data. G5.4.4 MatJabas River Assessment of the water quality for the MatIabas River is based on the data for Station A4H004, located 70 km upstream of the Matlabas/Limpopo River confluence. The data set contains 161 records and begins in 1971. Table G-A.2 shows the statistical analysis of the data, which reveal that the water is slightly acidic (50 percentile pH is 6.4) but suitable for domestic, agricultural, industrial and recreation use. From major ion analysis, the water should have no detrimental influence on aquatic life. From Table G-A.2, the mean annual runoff of the MatIabas River gives rise to 3% of the total tributary runoff, and less than 1% of the total tributary export of TDS. The Matlabas River contributes the lowest runoff and second lowest export of TDS. Trend analysis reveals no significant change in the major ion composition for the period 1971 to 1990. G5.4.5 Bonwapitse, Mahalapswe and Lotsane Rivers No water quality data are available for these tributaries, consequently it is not possible to determine the fitness of use, or TDS load contribution to the main river channel. Estimates of the mean annual runoff show these tributaries to have a comparatively minor runoff. In terms of the TDS export, it is assumed that the load is equally small. G5.4.6 Mokolo River Assessment of the Mokolo River is based on the water quality data for three stations; A4HOlO at Hans Strijdom weir 70 km upstream of the Mokolo/Limpopo confluence; A4R001 at Hans Strijdom Dam; and A4HOO7 on the Tambotie River which enters the Mokolo River downstream of Hans Strijdom Weir. Table G-A.1 shows the results of the statistical analysis for the three stations. Comparison of the statistics with the guideline concentrations reveals that the river is slightly acidic, due to low calcium buffering capacity, but suitable for domestic, agricultural, industrial and recreation use. From major ions analysis, the water should have no detrimental influence on aquatic life. From Table G-A.2, the mean annual runoff for the Mokolo River gives rise to 17% of the total tributary runoff, and 4% of the total tributary export of TDS. Trend analysis reveals that the major ion composition exhibited a G-42 5% increase in chloride ion concentration for the period 1984 to 1990. Such an increase may have been associated with evaporative losses from the Hans Strijdom Dam during drought years, or increased agricultural activity in the catchment. Similar trends are not found at other sampling stations in the catchment area. GS.4.7 Lepba\a\a River A detailed catchment study is being carried out by Chunnett, Fourie and Partners (CFP, 1990). It is reported that the water quality is good in terms of a low conductivity (less than 30 mS/m, equivalent to a TDS of about 200 mg/!). There are indications that the chloride ion concentration in the lower reaches of the river might influence sensitive crops. Assessment of the Lephalala River is based on the water quality data for station A5ROO2, located at Susandale Dam 30 km upstream of the Lephalala/Limpopo River confluence. Table G-A.l shows the statistical analysis of the data. Comparison of the statistical results with the guideline concentrations in Table G5.2 show the water is suitable for domestic, agricultural, industrial and recreation use. In terms of major ion analysis, the water should have no detrimental influence on aquatic life. From Table G-A.2, the mean annual runoff for the Lephalala River gives rise to 14% of the total tributary runoff, and 11 % of the total tributary export of TDS. Trend analysis of the major ion analyses shows no significant change in qUality for the period 1978 to 1990. GS.4.8 Mogalakwena River A catchment study is being carried out by SRK (1990), but at the time of writing no information was yet available from that source. Assessment of the water quality of the Mogalakwena River is based on the data for station A6H009, located 20 km upstream of the Mogalakwena/Limpopo River confluence. The station has an extensive data set consisting of 273 records for the period 1971 to 1990. Table G-A.1 shows the results of the statistical analysis of the data. Comparison of the statistical results with the guideline concentrations in Table G5.2 show the water is suitable for domestic, industrial and recreation use, as well as livestock watering. The irrigation guideline for chloride is exceeded at the 50 and 90 percentile limit so problems will be experienced when irrigating sensitive crops. This is confrrmed by calculating the SARIEC value which indicates a Class 2 water. From Table G-A.2, the mean annual runoff for the Mogalakwena River gives rise to 11 % of the total tributary flow, and 12% of the total tributary export of TDS. Trend analysis of the major ion analyses reveals negligible change in qUality for the period 1971 to 1990. GS.4.9 Motloutse River Assessment of the water quality is based on seven samples collected at points along the length of the Motloutse River (BNWMPS: SMEC, WLPU, SGAB, 1990). Unfortunately, only one sample was collected at the most downstream point nearest to the confluence with the Limpopo River. Comparison of individual results with the O"uideline concentrations show the water is suitable for domestic, agricultural, industrial and recreation uses. It '" G-43 is not possible to determine the influence of the water quality on aquatic life using only major ion analyses due to reports of mining effluent from BCL entering the river upstream (BNWMPS: SMEC, WLPU, SGAB, 1990). During periods of high runoff, the possibility exists that mining effluent will enter the Motloutse River. Such effluent is expected to contain high concentrati?ns of metals. From Table G-A.2, the mean annual runoff for the Motloutse River contributes 16% of the total tributary runoff, and an estimated 17% of the total tributary export ofTDS. GS.S WATER QUALITY ASSESSMENT: LIMPOPO RIVER Assessment of the water quality of the main river channel of the Limpopo River is based on the data for station A5H006. This station is situated at Sterkloop, approximately 300 km downstream of the Manco/Crocodile River confluence. This station represents a key water quality monitoring point as it receives runoff from a catchment 2 of 98 240 km , which includes the tributaries Marico, Crocodile, Notwane, Bonwapitse, Matlabas, Mokolo and Lephalala. An additional water quality monitoring point, A7HOO4, is located on the Limpopo River at Beit Bridge. Station A7HOO4 is unfortunately outside the target area and receives additional runoff from the Shashe River. Consequently, the data for A7HOO4 is discussed only briefly. The data set for Station A5H006 (SterkIoop) comprises 126 records sampled for the period 1980 to 1988. A statistical summary of the water quality data is shown in Table G-A.l. Comparison of the 90 percentile statistics with the guideline concentrations in Table G5.2 show the water is suitable for domestic, agricultural, industrial and recreation use. The river water should also have no detrimental affect on aquatic life, as assessed from the results of major ion analysis. In Table G-A.2 the statistical summary of the data for station A 7H004 also shows the Limpopo River at Beit Bridge is suitable for all the uses of water shown in Table G5.2. Table G-A.2 shows the calculated mean annual runoff and TDS export for station A5H006, which is 412 m31Q6 and 57 026 tons, respectively (see Row 11 of Table G-A.2). The summation of the tributary inflows which contribute to the runoff at SterkIoop is 515 m31~. The difference in runoff estimates is assumed to be due to in-channel losses. Summation of the TDS loads for each of the tributaries upstream of station A5HOO6 is 87 575 tons. Taking the in-channel losses of the upper tributaries into account, it is apparent that a proportion of the TDS load from the Notwane, Marico and Crocodile Rivers does not reach ASH006. Recalculating the TDS loads, accounting for a proportional reduction of the TDS load for the in-channel losses, gives an annual TDS load of 61 643 tons at station A5HOO6. This gives a difference in the TDS load estimates of 8%, which is acceptable in terms of the limited data sets. The load estimates shown in Table G-A.2 are used in Section G5.6 to determine the in-lake TDS concentration of the proposed impoundments a Buffelsdrift and Selika. GS.6 WATER QUALITY ASSESSMENT: PROPOSED RESERVOIRS The preliminary economic analysis, reported in Chapter J2 of Annex J, identified two groups of dam sites as a offering the greatest economic benefit. This initial assessment identified Buffelsdrift as the leadino contender from the Upper Group, with Selika preferred from the Middle Group. In common with other components of the Study the pre-feasibility level assessment of water quality has concentrated on these two sites, although results can also by cautiously applied to other dam sites in the same group. G-44 The water quality of the proposed impoundments at Buffelsdrift and Selika have been assessed in terms of their potential salinization and eutrophication potentials; two problems experienced in a large number of waterbodies in Southern Africa The modelling exercise has been carried out assuming that either Buffelsdrift or Selika is built, but not both. Because of the relative scarcity of data, the proposed darn at Ratho, from the Lower Group of sites was not modelled. GS.6.1 Buffelsdrift Dam The location of Buffeisdrift dam site is shown on Figure G5.1. The impoundment will receive runoff from the Notwane, Crocodile and Marico Rivers. For the purposes of mathematical modelling the impoundment has been 3 6 3 6 assumed to have a storage capacity of 750 m 10 and annual demand of 47.2 m 10 , equivalent to a 95% reliability yield, see Table G6.4. Flow balance The runoff entering the darn is obtained from the application of the Pitman Model which is calibrated and used to generate a monthly sequence of runoff for a period of 64 years (1924 to 1987). The method used to generate the runoff sequence is described in Annex F: Hydrology. The tributary runoff data was processed to account for the in-channel losses discussed in Section G5.5. Precipitation, evaporation and abstractions were calculated on a monthly basis and formed the volume loss from the impoundment. Groundwater seepage is assumed to be a minor loss and excluded from the flow balance calculations. Hence, in terms of the continuity principle the change in volume of the impoundment is given by: dV Idt = I - Q - E ...... 1 where: V = volume of impoundment I = inflow runoff Q = abstraction (including spillage) E = net evaporation (evaporation - precipitation) Mass balance for total dissolved salts Each of the flow components, with the exception of evaporation, may be a source of dissolved salts, a mass balance of which yields: dM/dt = d(Vc)/dt = LW - Qc ...... 2 where: M = mass of salts held in the impoundment c = salt concentration LW = summation of the mass influx of dissolved salts to the impoundment Qc = mass of salts in outflow (discharge x concentration). Equation 2 is suitable for dissolved conservative constituents, which are less influenced by stratification than non conservative or particulate substances (O'Connor, 1989). From the studies of Bosman and Kempster (1985), G-45 estimates were made as to the mass input of TDS to the impoundment from rainfall. An estimated mean TDS concentration of 11 mg/I was used in the calculations. A sensitivity test using a figure of 25 mg/I for the mean TDS concentration in the rainfall caused a 1% difference in the final TDS concentration. Hence, the mass balance is relatively insensitive to the input of salts from rainfall, and sensitive to the input of salts from rivers. Based on information obtained from studies of other impoundments in the basin (DW A, 1990), it is assumed that Buffelsdrift Dam will have minimal stratification with regard to total dissolved salts. The monthly runoff is used in conjunction with the regression coefficients for TDS concentration and runoff, shown in Tables G-AA and G-A.5, to determine the monthly TDS load entering the impoundment from the three inflows. The mass balance for the impoundment is calculated on a monthly basis and summed to give annual TDS concentration values for the 64 year period. Results of TDS mass balance simulation Figure G5.3 shows the results of the model simulation for Buffelsdrift Dam, where the TDS concentration is shown as a time series and as a percentage exceedance/non-exceedance diagram. Comparison of the data in Figure G5.3 with the guideline concentrations in Table G5.2 shows the water in the impoundment will be suitable for domestic use. Converting the P50 TDS concentration of the reservoir water of 500 mg/I into a conductivity value, using a conversion factor of 6.5, yields an EC of 76.9 mS/m, which is below the guideline concentrations for irrigation and livestock watering. However, as the conductivity is close to the limit of 80 mS/m there could be problems for irrigation of certain sensitive crops. Eutrophication The only point sOurces of phosphorus to enter the impoundment are from the Notwane and Crocodile Rivers. Due to the potential for soil erosion in the basin it is anticipated that diffuse sources of phosphorus will dominate the loading to the impoundment. Unfortunately, no total phosphorus analyses have been recorded so it is not possible to use a eutrophication model to estimate the in-lake chlorophyll concentration. From information obtained for other impoundments in the area it is anticipated that the turbidity of the impoundment will be sufficiently high to limit light penetration and hence limit algal growth (DW A, 1990) .. Aquatic weeds From the records of the DW A(RSA) there have been incidents of water hyacinth occurring in the lower Crocodile River. This subject is discussed further in Chapter H5 of Annex H. Effect on physicochemistry of the river downstream of the dam Due to the lack of available chemical, physical and biological data for the main river channel of the Limpopo it is not possible to make any conclusive statements with regards to the influence of the impoundment on the downstream physicochemistry of the river. O'Keeffe et al (1990) report that impoundments in middle reaches, such as Buffelsdrift, cause pronounced effects on downstream riverine conditions. Additional monitoring will be required to determine the impact of the impoundment. G-46 Figure GS.3 Simulated TDS Concentrations for Buffelsdrift Reservoir TDS concentration (mg/l) 800 ,volume (million m 3) 2000'1is------~~~------~ I le 1500r 600 l I 1000 ~ 400 L ! I 500 L 200i/ r O:L!~----~~--~~~~~~~ )~. o 10 20 30 40 50 60 70 80 90 100 o 10 20 30 40 50 60 70 80 90 100 Percentage non-exceedance Percentage Non-exceedance 3 TDS concentration mg/l Dam vohllne (million m. ) 2000,' ------" 800 lA I I - [TDS] mg/l Dam volume j I 1500 L ~ /1 ~600 I Ii I I \ ! 1000 ~ ;/\J rv' ).I I i ~ 400 I . \ I \ I i I I' '. \. l \ I lA 200 V . /. . V ~ I I I 0 1 0 1924 1934 1944 1954 1964 1974 1984 Year G5.6.2 Selika Dam The location of Selika Dam is shown in Figure GS.1. For modelling purposes an impoundment capacity of 1 250 m3106 has been assumed, with an annual demand of 88.5 m3 106, equivalent to the 95% reliable yield, see Table G6.4. Flow balance The impoundment will receive runoff from the Notwane, Bonwapitse, Marico, Crocodile, Matlabas, Mokolo and Lephalala Rivers. From the river modelling simulations, the inflow to Selika Dam was determined taking account of in-channel losses. Equation 1 is used for the volume balance for Selika Dam and takes into account evaporation, precipitation and spillage volumes. Mass balance for total dissolved salts Equation 2 is used to simulate the in-lake TDS concentration. To calculate the mass inflow of TDS to Selika Dam the TDS concentration/river discharge relationship for station A5H006 (Sterkloop) is used. The data set for this station is selected as it represents the combined water quality of the various tributary inflows at a point on the Limpopo River in close proximity to the Selika Darn site. Results of TDS mass balance simulation Figure G5.4 shows the results of the model simulation for Selika Dam. Comparison of the TDS concentration data in Figure G5.4 with the guideline concentrations in Table GS.2 show the water will be suitable for domestic, irrigation, livestock watering, industrIal and recreation use. Eutrophication As discussed in the previous section the only point sources of phosphorus in the upper catchment are along the Notwane and Crocodile rivers. From the incremental catchment between Buffelsdrift and Selika dam sites the only potential source of wastewater effluent to enter the impoundment is from the Mokolo catchment. The re-use of Ellisras municipal and industrial effluents has resulted in minimal phosphorus being discharged to the surface waters. It is therefore anticipated that diffuse sources will be the major contributor of nutrients entering Selika Dam. The data set for A5H006 (Sterkloop) does not include total phosphorus so it is not possible to use a eutrophication model to estimate algal growth in the impoundment. From information obtained for other impoundments in the area it is anticipated that the turbidity of the impoundment will be sufficiently high to limit light penetration and hence limit algal growth (DWA, 1990). Aquatic weeds From the records of DW A(RSA) there have been incidents of water lettuce (Pistia stratiotes) occurring in the Limpopo River and Parrots feather (Myriophyllum aquaticum) in the Mokolo River. This subject is discussed further in Chapter HS of Annex H. G-47 GS.7 CONCLUSIONS GS.7.1 Tributaries and Main River Channel or the Limpopo River In terms of fitness for use, it is found that the main river channel and all tributaries are suitable for domestic use, livestock watering, industrial use and ~ecreation. Irrigation from the lower reaches of the Crocodile and Mogalakwena Rivers may cause problems for sensitive crops. The Notwane and Motloutse Rivers both receive urban and industrial effluents which could influence the quality of the main channel of the Limpopo River. The lack of available water quality data has precluded a satisfactory assessment of the Notwane, Bonwapitse, Lotsane and Motloutse Rivers. GS.7.2 Proposed Impoundments at Buffelsdrift and Selika At the Buffelsdrift Darn site, the water quality shows considerable variation due to evaporative losses and relatively high TDS load from the Crocodile and Notwane Rivers. Unfortunately, the lack of data for the Notwane River makes it essential to verify the TDS concentration simulation for Buffelsdrift Darn with more detailed data for the various inflows. At the minimum supply level, the impoundment is expected to have a TDS concentration of between 500 and 600 mg/i, and at the full supply level a TDS concentration of between 300 and 400 mg/i, once the pattern is established after the initial ftlling period. From these simulations it is predicted the water will be suitable for domestic, livestock watering, industrial and recreation. However, it is possible for problems to occur in the irrigation of chloride sensitive crops during draw-down of the impoundment. Due to the lack of available data it is not possible to make an assessment of the eutrophication potential of the impoundment. Should the impoundment become eutrophic, water treatment costs would be greatly increased. At the Selika Darn site the long term picture can be summarised as a relatively high TDS concentration of between 500 and 600 mg/I during periods of low capacity and a lower TDS of between 200 to 300 mg/i at full supply capacity. The 50 percentile TDS concentration is 200 mg/I, which is suitable for all water uses. Insufficient data are available to make conclusions with regard to the eutrophication potential of the impoundment. The improved water quality at Selika Darn, compared with Buffelsdrift Dam, is due to the low TDS export from the tributaries in the mid-basin. As yet there_ are no data available to assess the present conditions of the aquatic envirorunent at, or near, the darn sites. It is essential that chemical, physical and biological data are collected from the river channel and riparian zone of the Limpopo to assess the possible influence of the impoundments on the downstream quality of. the river. GS.S RECOMMENDATIONS FOR FURTHER WATER QUALITY MODELLING AND MONITORING IN THE BASIN GS.S.l Water Quality Modelling From the preliminary model simulations described in Section G5.6 it is essential that further water quality modelling work is undertaken to: G-48 Figure G5.4 Simulated TDS Concentrations for Selika Reservoir 1000 TDS concentration (mg/l) B ::::fume 800 1000 600 800 400 600 400 200 OL-~--~~~--~~--~~--~~ o~~--~~~--~~--~~--~~ o 10 20 30 40 50 60 70 80 90 100 o 10 20 30 40 50 60 70 80 90 100 Percentage non-exceedance Percentage non-exceedance 3 [TDS] (mg/l) Volume (million m ) 1000 1400 A - [TDS] Volume 1200 800 1000 600 800 '; 600 400 400 200 200 0 0 1924 1934 1944 1954 1964 1974 1984 Year • Verify the TDS mass balance simulations for the proposed impoundments. • Undertake phosphorus modelling of the basin to determine the eutrophication potential of the impoundments. • Determine the impact of the impoundments on the water quality of the downstream reaches, with reference to the conservation of the riverine ecology, riparian vegetation, and alluvial aquifer system. • Develop a water quality monitoring system to provide information for the effective management of the impoundments and rivers. G5.8.2 Water Quality Data Collection In order to carry out the above mentioned water quality modelling investigations, it is recommended that the following data collection procedure be adopted with immediate effect so that for Stage II studies, a record of one hydrological year is available. (1) Water quality samples should be collected at the location points shown in Figure GS.1. The sites on the tributaries have been selected close to their confluence with the Limpopo, at road crossings where access will not be a problem. (2) Water samples collected from the tributaries of the Limpopo river should be analysed for: • major ions • turbidity and total suspended solids (TSS) • KN and 1P Water samples collected from the three points along the Limpopo river should be analysed for: • major ions • turbidity and total suspended solids • KN and 1P • temperature • dissolved oxygen (DO) (3) The water samples for the sampling points on the tributaries and main channel of the Limpopo river should be sampled on a weekly basis for a full period of one year. The transient flow characteristics of the rivers in the basin will require the collection of samples more frequently than once a week during flood events. (4) From the beginning of the rainy season, samples should be collected on a monthly basis for trace metal and E.Coli analysis at the tributaries, and trace metals and chlorophyll-a at the points on the main river channel. Table G5.3 shows the proposed water quality sampling requirements for Stage II of the study. G-49 (5) It is recommended that a gauging plate be installed at the proposed sampling points shown in Figure G5.1 to enable the estimation of river discharge. It would also be necessary to develop a rating relationship between gauge plate height and river discharge. Table GS.3 Proposed Water Quality Sampling Requirements Location Frequency Variables 5 Tributaries (RSA) Weekly Major ions, turbidity, TSS, KN, TP, Monthly E.Coli, trace metals 3 Tributaries (Botswana) Weekly Major ions, turbidity, TSS, KN, JP, Monthly E.Coli, trace metals 3 points on the Limpopo River Weeldy Major ions, turbidity, TSS, KN, TP, Temperature, DO Monthly Trace metals, Chlorophyll-a Estimated number of samples for: Major ion analysis = 176 Turbidity = 176 E.Coli = 32 Trace metals = 44 Chlorophyll-a = 12 GS.8.3 Comments on Recommendations Both DWA(RSA) and DWA(Botswana) have commented that the recommendations outlined above appear excessive. Therefore, it will be necessary to undertake detailed discussions on the proposed programme at the commencement of Stage IT studies to identify the most crucial requirements for the pwpose of the study and as a basis for long term monitoring. G-50 CHAPTER G6 RESERVOIR YIELDS G6.1 INTRODUCTION The initial reservoir yield analysis covered the 25 dam sites identified by the reconnaissance assessment of possible sites, see Annex D, Chapter D4. These initial assessments were made on the basis of the river flow sequences derived during the earlier LWUS (MacDonald, 1987) on the basis of a 95% reliability criterion: the results are presented in Section G6.2. As a result of this initial assessment, eleven sites were selected for more detailed analysis to a pre-feasibility level. The eleven sites have been considered as three sets, an Upper Group comprising Cumberland, Buffelsdrift and Riversdale, a Middle Group comprising Sunnyside, Selika, Martins Drift, Worcester and Graaf Reinet and a Lower Group inCluding Mopani, Ratho and Ponts Drift, see Figure G 1.1. Yields have been established for these eleven sites under a range of flow sequences, reliability criteria and development options. An assessment has also been made of the yield from an offstream storage site to be operated in conjunction with one of the more promising dam locations on the main stem river. The derivation of the inflow sequences is reported in Annex F. These take account of tributary inflows and losses along the length of the river to establish a 64 year (1924 to 1987) flow sequence at each dam site. This process has been followed to simulate a number of different hydrological regimes, namely: • base case hydrology (assuming 60% of unaccounted losses occur within the study area) • alternative flow set 1 (assuming 100% of unaccounted losses occur within the study area) e alternative flow set 2 (assuming 40% of unaccounted losses occur within the study area) base case hydrology modified by Cumberland dam storing 750 m3106 • . base case hydrology modified by Buffelsdrift dam storing 750 m31Q6 • base case hydrology modified by Selika dam storing 1 QOO m3106 • hydrology as modified by possible future developments along the tributaries • hydrology reflecting the level of development and water use in 1980 The yield assessments have been made for a number of different reliability criteria. Reliability is generally assessed in terms of the proportion of months that the specified yield is delivered (using a particular flowset), the exception being the availability of water for irrigation, which is assessed on the basis of the number of successful crop seasons. The range of yield criteria considered is as follows: • 100% reliability criterion - full supply always available • 95% reliability criterion - full supply for 95% of months • 80% reliability criterion - full supply for 80% of months (for irrigation supply) • 95/99% reliability criterion - full supply for 95% of months, 80% of full supply for a further 4% of months (for urban supply) • 80/95/99% reliability criterion - multi~use with full irrigation supply for 80% of crop seasons and urban supply at the 95/99% reliability criterion G-51 The fxrst two reliability criteria were intended to provide some idea of the relative yields of the various dam sites. The 80% criteria was used to establish the amount of water available for inigation use. The practical effect of this criterion is to not supply any water during the remaining 20% of months. Given the nature of the flow regime of the Limpopo these months of shortfall are likely to occur in groups, possibly extending over a number of years. The impact on cash flows of such an extended period of inactivity would have serious implications with regard to the viability of irrigated farming. An alternative approach would be to adopt a lower reliability for full supply but to ensure that a minimum is available for the most of the remaining period. The total releases over a given period as a result of such an operation policy are likely to be similar to those resulting from the adoption of an 80% reliability criterion. As inigation benefxts are closely related to the total volume supplied it is considered that the adopted 80% criterion is suffxciently representative to assess the viability of inigation at the pre-feasibility level. However, the issue of reliability criteria for assessing irrigation potential should be considered more fully during the Stage II studies. The 95/99% reliability criterion for urban supply is the standard adopted for assessing resources in Botswana. It should, however, be noted that no account is taken of any possible additional yields as a result of the conjunctive use of a number of resources, for which the critical periods may not necessarily completely coincide. In the remaining 1% of months not covered in the 95/99% definition there is no minimum delivery specifxed, and so there may be no supply. It must be appreciated that the existing system already has a yield, as a result of the numerous irrigation weirs along the length of the Limpopo. An assessment of yields from these weirs is important from a view point of compensation releases (no farmer should be significantly worse off after project implementation) and the economic analysis (compensation releases do not represent a net benefxt attributable to the project). Yields from the existing system have been estimated for a range of different hydrologies and development options; the results are presented in Section G6.4. As a fmal analysis a preliminary assessment has been made of the reliability of supplies from selected dams through the early years, immediately following commissioning. The results of the basic assessment of all sites for a single darn and single use are presented in Section G6.3, with the analysis of the multi-use option presented in Section G6.5. It is possible that one dam could be developed from each of the three groups. The yields resulting from such a multi-dam development are presented in Section G6.6. The assessment of the potential increase in yield as a result of the use of the Shapane offstream storage site in conjunction with a darn at Selika is covered in Section G6.7. The analysis of reliability in the years immediately after commissioning discussed in Section G6.9. Section G6.8 is devoted to an assessment of yields under the future development scenario. All yield assessments have been computed by the Consultant's existing analysis software. This adopts a monthly time step for inflows, with losses due to evaporation and rainfall inputs based on the average reservoir area for the month. Open water evaporation is held constant from year to year, with the variation within the year defxned by a set of monthly estimates. Monthly rainfall for the 64 year Period is based on the closest extended composite rainfall record developed for the catchment modelling studies. The initial conditions are set by assuming a three year period of average inflows starting from an empty reservoir, with the assumed demand pattern drawing from the reservoir - these three years are excluded from the assessment of reliability. Demand at the given reliability is derived by a trial and error process, with trigger levels used to establish yields with components at different reliabilities. G-52 G6.2 INITIAL YIELD ESTIMATES Estimates of yield were made for all 25 dam sites identified by the reconnaissance survey, to enable an initial comparison to be made between the sites. As no reprocessed hydrological records were available at the time recourse was made to the flow records published in L WUS (MacDonald, 1987). These records had also formed the basis for the BNWMPS, although the record had been extended to include the hydrological years 1924, 1986 and 1987. LWUS developed two flow sequences, a "high" and "low" flow series: in all cases the low flow sequence was adopted. Three flow sequences were prepared, corresponding to the three dam sites considered in LWUS, namely Cumberland, Martins Drift and Pants Drift. In order to derive inflow sequences for each of the 25 sites under consideration, a factor was applied to adjust the MAR of the nearest of the three flow sequences to that of the site. The MAR at each site was derived from the "developed MAR" for each tributary, as reported in LWUS, with a simple adjustment to the MAR to bring it into line with the figures given in LWUS. In order to provide a common reference system all dam sites and tributaries were fixed by a chainage along the river, with the origin taken at the confluence of the Marico and Crocodile. The full list of the features along the river is given in Appendix B-B of Annex B. The basic data for the derivation of MAR for each reach of the river is presented in Table G6.1. Table G6.1 MAR Assumed for Preliminary Estimates of Yield Tributary/location Chainage Tributary MAR Limpopo MAR Interpolated MAR (km) (m3106) (m3106) (m3 1 The results of the preliminary yield analyses for- the 25 dam sites are presented in Table G6.2. A 95% yield reliability criterion was adopted, with yields derived for a range of reservoir capacities at each site. The G-53 shortcomings of this preliminary analysis were recognised, but the approach was considered appropriate for the preliminary appraisal of the large number of sites. In particular the 95% yield criterion significantly overestimates the amount of water available to meet urban demand, which is established on the basis of a 95/99% reliability criterion. Table G6.2 Preliminary Yield Analysis for Limpopo Dam Sites Chainage Dam site Estimated Yield at 95% reliability (km) MAR Reservoir storage (m31(r) 250 500 750 1000 1250 1500 2000 35.10 Cumberland 182 15.5 23.6 28.0 32.0 34.2 34.1 34.2 61.20 Buffelsdrift 219 17.2 33.1 39.9 43.5 46.7 46.5 46.4 71.70 Riversdale 252 15.6 31.0 37.7 43.0 48.2 48.3 48.3 138.65 Holmlea 258 14.0 24.8 28.9 33.0 36.6 40.3 44.0 152.60 Exchange 279 14.2 24.5 30.2 33.5 36.9 40.7 45.9 177.25 Parrs Halt 279 15.8 32.4 39.9 45.7 50.1 53.5 54.7 221.45 Marnkalalo 279 21.4 39.2 46.3 51.8 55.2 58.6 61.0 271.70 Sunnyside 406 44.0 74.1 98.9 114.8 122.4 132.2 140.6 290.75 Selika 479 37.0 67.6 93.6 113.5 127.3 138.8 153.8 304.15 Martins Drift 479 37.8 62.9 85.1 102.3 112.0 118.7 130.4 317.70 Worcester 479 40.3 69.6 95.6 113.5 127.6 136.6 152.3 337.00 Graaf Reinet 479 42.7 72.5 101.4 120.1 136.7 147.2 165.0 353.30 Geluk 495 48.4 85.1 119.9 142.6 159.6 174.3 191.4 367.50 Benedict 495 48.3 84.5 120.1 143.1 159.9 176.2 187.0 376.10 Bambata 495 50.3 87.3 120.3 140.6 155.9 165.5 182.1 386.00 Zanzibar 495 55.5 92.2 127.4 150.2 166.3 180.5 196.8 390.30 Dunsandle 495 53.0 92.0 128.5 151.9 168.5 184.9 202.2 398.55 Umgeni 495 61.8 104.0 140.4 163.1 180.4 197.4 214.8 408.20 Bains Drift 495 55.9 100.1 140.4 169.0 188.0. 204.7 219.2 418.45 Marapong 495 48.5 89.6 ·128.7 155.5 174.6 193.0 * 433.25 Tswehe 495 48.7 87.0 124.2 149.4 166.9 183.8 * 440.00 Shangai 575 61.4 96.8 134.6 167.0 188.0 205.8 227.5 451.30 Mopani 575 66.5 103.6 142.5 174.1 197.5 212.8 237.3 470.80 Ratho 630 57.5 95.0 133.3 170.9 195.6 215.7 244.6 476.00 Ponts Drift 630 51.0 86.4 121.4 158.5 183.7 202.2 232.4 G-54 G6.3 BASIC YIELD ANALYSIS The yields presented in Table G6.2 fonned the basis of the initial ranking of dam sites, as discussed in Chapter J2 of Annex J. This lead to the identification of 11 sites for further study, in three groups, covering the upper, middle and lower reaches of the Limpopo in the study area All subsequent yield analyses have concentrated on the selected sites. The first task of the more detailed yield analyses was to evaluate the impact of the updated infonnation relating to hydrology, rainfall, evaporation and reservoir basin characteristics. As discussed in Chapter F5 of Annex F, sufficient doubt remained about the quality of the available infonnation to suggest the generation of three different sets of flow sequences, allowing for different interpretations and assumptions. It was crucial to detennine the effect on yields of these alternative flow series. G6.3.1 Base Case Flow Series The hydrological studies, reported in Annex F, established a "base case" flow series, for each of the dam sites. The resulting current development level MAR's are summarised in Table G6.3, along with the MAP and evaporation estimate for each site. Table G6.3 Summary of Parameters for the Base Case Yield Assessment Chain age Dam site MAR MAP Evaporation (km) (m31Q6) (mm) (mm) 35.0 C umberl and 230.1 417 1 850 61.2 Buffelsdrift 254.4 425 1 850 71.7 Riversdale 275.2 425 1 850 271.7 Sunnyside 340.9 399 1 835 290.8 Selika 412.1 380 1800 304.2 Martins Drift 412.1 380 1 800 317.7 Worcester 412.1 377 1 800 337.0 Graaf Reinet 365.3 377 1775 451.3 Mopani 287.5 382 1 785 470.8 Ratho 403.5 389 1 785 476.0 Ponts Drift 403.5 389 1 785 The base case yield analysis was undertaken for all dam sites, using the same range of reservoir capacities as covered by the initial studies, for the 95% yield reliability criterion. The process was repeated using the 100% and 80% reliability criteria. Yields were also established for the 95199% criterion for the upper and middle group of sites. although this was not extended to the downstream sites as they were not viable contenders for meeting urban demand. The results are presented in Table G6.4. G-55 Table G6.4 Base Case Yields for Selected Dam Sites Chainage Dam site Reservoir storage (m 3106) (km) 250 500 750 1000 1250 1500 2000 95% reliability 35.1 Cumberland 9.9 25.6 35.7 42.0 46.4 49.1 51.8 61.2 Buffelsdrift 19.4 34.9 47.2 56.4 63.2 67.8 69.8 71.7 Riversdale 22.4 42.2 57.3 69.3 76.9 83.9 88.9 138.7 Sunnyside 19.4 38.0 58.4 74.7 87.9 93.4 105.0 290.8 Selika 30.4 47.6 63.4 76.9 88.5 100.0 117.7 304.2 Martins Drift 31.6 47.9 62.6 76.2 86.7 97.8 111.8 317.7 Worcester 36.9 56.0 73.1 87.2 99.2 110.6 126.5 337.0 Graaf Reinet 30.7 52.3 71.8 87.2 102.2 116.1 129.4 451.3 Mopani 45.5 69.3 92.8 114.5 125.7 127.1 127.0 470.8 Ratho 86.2 127.2 159.5 181.4 201.4 213.9 234.9 476.0 Ponts Drift 78.4 119.5 149.1 173.8 191.0 204.0 223.6 100% reliability 35.1 Cumberland 2.9 10.1 18.9 22.5 22.3 22.3 22.5 61.2 B uffelsdrift 8.2 15.5 25.3 32.0 32.0 31.9 31.7 71.7 Riversdale 10.1 19.8 30.9 43.0 44.3 44.3 44.4 138.7 Sunnyside 9.0 22.6 32.2 43.0 54.0 60.4 60.1 290.8 Selika 9.5 23.8 38.1 51.0 62.0 68.5 76.2 304.2 Martins Drift 10.6 24.4 38.5 50.8 60.1 65.9 75.2 317.7 Worcester 14.7 31.4 48.0 . 62.0 73.2 79.8 85.0 337.0 Graaf Reinet 14.0 34.7 50.5 60.8 68.42 76.91 78.1 451.3 Mopani 24.3 51.5 67.6 79.4 80.3 80.21 80.1 470.8 Ratho 40.2 76.3 111.9 140.6 166.6 171.4 171.4 476.0 Ponts Drift 32.8 66.4 102.4 130.l 155.3 161.1 161.1 G-56 Table G6.4 (cont) Chainage Dam site Reservoir storage (m 3106) (km) 250 500 750 1000 1250 1 500 2000 80% reliability 35.1 Cumberland 36.0 64.2 82.0 91.3 98.9 108.4 120.1 61.2 B uffelsdrift 56.6 82.6 104.2 116.0 126.9 137.3 150.4 71.7 Riversdale 61.1 92.5 117.9 133.5 145.0 154.2 175.3 138.7 Sunnyside 52.9 94.1 123.8 146.0 167.8 180.5 207.0 290.8 Selika 81.9 120.0 146.3 173.6 200.2 216.4 244.1 304.2 Martins Drift 83.0 120.7 145.9 171.9 197.8 213.3 235.4 317.7 Worcester 89.9 128.4 157.3 183.8 210.3 227.0 250.5 337.0 Graaf Reinet 73.1 110.5 142.6 176.0 198.0 219.7 252.0 451.3 Mopani 91.6 137.8 173.0 196.0 215.8 232.5 255.9 470.8 Ratho 169.8 232.3 275.2 307.6 324.4 350.1 377.4 476.0 Ponts Drift 163.9 222.9 264.2 296.1 319.2 338.4 366.2 95/99% reliability 35.1 Cumberland 4.6 14.6 25.5 29.3 29.6 30.0 30.3 61.2 Buffelsdrift 12.3 22.0 34.1 41.0 41.8 42.5 42.5 71.7 Riversdale 14.2 26.7 40.3 54.2 55.2 56.0 56.5 138.7 Sunnyside 12.8 26.6 41.0 54.7 68.4 74.0 75.2 290.8 Selika 15.4 31.1 46.7 60.1 70.2 79.5 88.9 304.2 Martins Drift 16.4 31.6 46.9 59.3 69.0 78.7 85.9 317.7 Worcester 21.3 39.4 56.6 70.6 81.8 89.4 98.6 337.0 Graaf Reinet 19.9 40.6 57.7 71.5 83.0 94.1 95.2 The yields for Buffelsdrift, Selika and Ratho are presented in Figures G6.1, G6.2 and G6.3, respectively. These figures clearly show the greater yield associated with the sites further downstream. Figure G6.1 highlights the substantial difference between the yields at an 80% reliability and those for higher reliabilities over the full range of reservoir capacities. This is as a result of the high variability in annual flows. It can also be seen that the 95/99% reliability yields are close to those resulting from the adoption of a 100% reliability criterion. Given the generally flat nature of the reservoir basins there is a significant penalty in terms of evaporation loss in ensuring adequate water is kept in storage to meet demand through the extended drought periods. Figure G6.1 also shows a comparative improvement in yields compared with the initial estimates (see Section G6.2). As discussed in Section D5.3 of Annex D, the revisions to the reservoir characteristics, as a result of updated basin mapping, were not significant in the case of Buffelsdrift and, therefore, the improvement in yields is essentially the result of the revised inflow series, with the MAR increasing from 219 m3106 to 254 m3106• G-57 The yields from Selika Dam (Figure G6.2) show a similar pattern to those at Buffelsdrift. although the range in yield in somewhat reduced, with the ratio of the 80% yield to 100% yield dropping to 3.3 for the 1 000 m3106 reservoir, compared to a ratio of 3.6 for Buffelsdrift. This is due to the improved reliability of flow as a result of a greater number of tributaries in the catchment of Selika. The figure also brings out the reduction in yield compared with initial estimates. Two factors have contributed to this; the first is a general flattening of the reservoir basin (see Section D5.3); the second is as a result of the revised hydrological assessment, with MAR reducing from 479 m3IQ6 to 412 m3IQ6. An investigation of the impact of each of these factors showed that each contributed almost equally to the reduction in yield. Figure G6.3 shows the yields at Ratho to be much higher than those at Selika. Another feature of this site is that the higher reliability of flow has reduced the relative difference between yields at different reliabilities, with the ratio of 80% to 100% yield only 2.2 for a 1 000 m3106 capacity reservoir. The comparison with the initial assessment of 95% yield shows the estimates to be reasonably close, in spite of a reduction in MAR from 630 m3106 to 403 m31Q6 as a result of the subsequent studies. As no adjustments have been made to the reservoir characteristics this must be as a result of the improved reliability of annual flow compared to the flow sequences derived in L WUS. G6.3.2 Alternative Flow Sequences Section F5.4 of Annex F discusses the basis for the generation of alternative flow series which were developed to gain some insight into the possible significance of different assumptions concerning river losses. This has resulted in two additional flow sequences for each reservoir, which have been used to establish yields over the range of reliability criteria. No adjustment has been made to rainfall or evaporation estimates. The MAR's for each site are summarised in Table G6.5, along with those relating to the "base case" flow sequences. Table G6.S Comparison of MAR's for Alternative Flow Sequences Chainage Dam site . MAR (m31Q6) (km) Base case Alternative 1 Alternative 2 35.0 Cumberland 230.1 263.6 221.4 61.2 Buffelsdrift 254.4 287.9 245.6 71.7 Riversdale 275.2 308.4 266.6 271.7 Sunnyside 340.9 337.0 350.5 290.8 Selika 412.1 ·411.0 412.5 304.2 Martins Drift 412.1 411.0 365.3 317.7 Worcester 412.1 411.0 365.3 337.0 Graaf Reinet 365.3 351.8 373.6 451.3 Mopani 287.5 202.6 348.8 470.8 Ratho , 403.5 315.9 466.4 476.0 Ponts Drift 403.5 315.9 466.4 Buffelsdrift Dam Reservoir Y iclds 240 220 - _. -.:~ - --- ···r········. ... . 200 180 160 ~ ...... 140 5 '"0 Q) 's:. 120 "@ :::l t:: t:: 100 . -< 80 60 40 ...... --< (lj 20 .~ 0- C/'] ~ o 0 o 250 500 750 1000 1250 1500 1750 2000 3 Reservoir storage (m!J 0") tJj c Legend ::p (lj' 100% reliability >-' C/l 95'Yo reliability 0- """I 'Tl 80% reliability ~. _...... ,.r-t-, (J~c:: 95/99('10 reliability t:I ;:; 95% initial estimate 8 ~ ;:j Selika Dam Heservoir Yields 240 ----: --:, - 220 : -----~------r------+--:r-~--:::------(------ 200 180 160 r"' '6 140 ~E '--' '0 ~ 120 >. c;a •• T.. -•••••.. ] _.:-.-.. •••• .. .. : ---. --- -- ::> -----:~J~ .~ ~ -~-~- r_~__ --~~"~~-·-,r··-. ~ \:: \:: 100 80 60 / : .," ~ t' I., .... I )_,~~~-~-;-.:-<'-~------~--- 40 _. ______._/_L ______. ------i~ -.------(------, !,.. ... " / , , , " , I _____ ,_.,..."t'." ______.~-_-_- 20 -----f: ------~ ------i / ,'...... --, ; ". I ."..::--___ : . _. - >-<: # .- (1) o ...... CL o 2S0 soo 750 1000 12S0 ISOO I"lSO :W()() (/J >-l) Reservoir storage (m} I 0') '"1 0 Legend 3 100% reliability C/) (1) 95% reliability ;:::-:'11 80% reliability 7;'" 0;' 9)/99% reliability ~ ..,c: 95(10 initial estimate Ci (~ r' Cl ,.-' C'\ 3 tJ Ratho Dam Reservoir Yields 240 :,... --- 220 _I ...... ;~ ...... - .... -- ...... - --'- ....< -~ ...... - ...... --_ ...... --_ ...... -:-;...... -- ...... ".~ ; ...... -_ ...... -_ ...... -- ;~ ...... -- ...... -- ...... ---7-"""" ; ; I: : : ; - -- I ., , : /:: : ..., . 200 ...... f········· -;!.... ·······f· ...... ·············f························f··················;.·.; -~. ~.- -.~.~.~ ...... +·······················f························ : I: :: :: : I : : : - - - : 180 ._------.---- ..... -... -.-- -r" -)" ------_. --_. ---t ------·------r----· ---._-- .. -.. ---:.:yf- .".-...... ------_ .. -_ .. --." ----r-- .. ·-- .. ------..... ------~ .-- -_ .. ------_ .. -.- --- .. -- :r :· :.. ... I .., ..,. , /! : : _/ :. ---- _.. -- .. -. ------_ ... ~ ... ". ----.- ... ". ------... --~ ----- .. ------;r--·· -_ .. ------" -- .. --.- .. -_ .... -- ...... --- .. -~------~--- _. ~ -~.- -~~ -- -- .------. 160 . . , . " I • , .., f " ~ '6 ..... 140 ...... f·i··· ..... ···i>/~ ... ~ ..... ~ ... ! ...... r·· ...... f························ E '-" :~... : :: "Cl I I .., , I " 0) I , • • I. 120 ---.------i------r -----~------~-;".. ---r------°r------~------.------: ---" ------r ------"» M I 1 ..,"': : :: c::::> c:: 100 ...... 1·········f····; __·,!·~~···········f.. ······+·······················f························ ·······················-f-·······················f······ ...... < :..,I .., :• " ~- . 80 ...... ·t···· ..... ;,.[...... , .~ ...... ' .. '" ..... ~ ...... ··f·· ...... ~ ...... "...... f ' I I 'I , I I t I I • I. ,. : :: ': I , : . :: .: 60 -- --_.. "i- ---.. _.. ; ,------~ ------f------_ .. ------r ------" ------" -----·"r ------_.. ----- _.. _ .. ------.. -----f" -----_.. -- _. ------._. -r _.... ----.- ---_. _.. -. _. -- I , " • t I ' : :: :. I : :: : 40 ·····f····,!···· ...... ··i ...... ·············t························r ...... ·i························~························ · . . 20 .. ~ .. -...... :-...... ··r "...... -···r...... "...... "...... --< (\) o >-' 0. o 250 500 750 1000 1250 1500 1750 20W C/J Reservoir storage (ml \06) o~ Legend 3 100% reliahility :;0 p 95% reliability &::E1 o (JO< 80% reliability C 95% initial estimate tJ (:J 5 'JJ~ The three different models of the hydrology were all calibrated against the flow records derived from the gauging weirs at Oxenham (km 309.8) and Sterkloop (km 312.3). Table G6.5 highlights the pivotal nature of these records with the different assumptions embodied in the alternative flow sequences making little difference to the derived inflow series for the middle five sites. As a preliminary exercise yields were assessed for the 95% reliability criterion, for the middle five sites, using both alternative flow series. The results are presented in Table G6.6 in terms of the base case yield estimates. Table G6.6 Comparison of Yields for Alternative Flow Series for the Middle Group of Sites 95% yield as a proportion of base case yield Chainage Dam site (km) Reservoir storage (m 3106) 250 500 750 1000 1250 1500 2000 Alternative flow series 1 138.7 Sunnyside 97.9% 99.5% 99.8% 100.1% 101.6% 102.8% 101.5% .290.8 Selika 104.3% 104.6% 104.7% 104.8% 104.2% 104.1% 102.8% 304.2 Martins Drift 103.8% 105.2% 106.1% 104.5% 104.6% 103.3% 105.2% 317.7 Worcester 104.6% 102.3% 104.1% 105.0% 103.3% 104.5% 103.6% 337.0 Graaf Reinet 104.2% 101.9% 103.3% 102.8% 102.8% 100.6% 101.0% Alternative flow series 2 138.7 Sunnyside 100.5% 100.3% 100.5% 100.1% 99.1% 101.9% 101.4% 290.8 Selika 94.1% 93.7% 95.1% 95.4% 96.4% 96.2% 95.7% 304.2 Martins Drift 94.6% 93.7% 94.9% 95.7% 96.2% 96.2% 96.3% 317.7 Worcester 93.0% 94.1% 95.2% 96.0% 95.9% 97.4% 96.4% 337.0 Graaf Reinet 92.5% 95.2% 97.1% 96.9% 98.1% 99.1% 99.3% The results given in Table G6.6 show that the small differences in MAR are reflected in the yields for the middle five sites, where the vast majority are within 5% of the base case. As this is well within the accuracy of the current level of estimates, it is concluded that the alternative flow series are not significantly different from the base case for the Middle Group of dam sites: consequently no further consideration has been given to alternative flow sequences for this group. Alternative Flow Series 1 assumes that 100% of the apparent losses occur within the target area, mainly the Limpopo itself (see Annex F, Section F5.4). The base case flow sequence is based on an estimate of 60% of unaccounted losses occurring within the target area, with the balance made up by reducing flows in the Crocodile (down by 18%) and Lephalala (down by 8%). The effects of these different assumptions on yields for the full range of reliability criteria are shown in Table G6.7. G-59 Table G6.7 Comparison of Yields for Alternative Flow Series 1 Yield as a proportion of base case yield Chainage Dam site Reservoir storage (m 3106) (km) 250 500 750 1000 1250 1500 2000 95% yield 35.0 Cumberland 115.2% 110.2% 112.0% 116.0% 116.6% 117.7% 121.0% 61.2 B uffelsdrift 107.7% 107.4% 109.1% 114.7% 113.6% 116.1% 118.2% 71.7 Riversdale 106.3% 101.9% 106.8% 108.2% 110.7% 110.1% 113.7% 451.3 Mopani 96.9% 95.8% 90.9% 81.9% 74.6% 73.8% 73.9% 470.8 Ratho 97.3% 98.6% 96.6% 93.8% 90.3% 90.8% 84.8% 476.0 Ponts Drift 97.8% 99.1% 97.7% 92.1% 90.5% 90.4% 84.3% 80% yield 35.0 Cumberland 117.2% 112.6% 113.2% 115.8% 118.2% 113.6% 115.6% 61.2 Buffelsdrift 106.7% 107.9% 107.3% 111.9% 112.6% 1l0.4% 112.6% 71.7 Riversdale 108.7% 111.0% 108.1% 108.0% 108.3% 110.1% 109.5% 451.3 Mopani 96.9% 93.5% 85.5% 83.2% 83.0% 78.1% 72.3% 470.8 Ratho 98.4% 93.6% 91.6% 87.7% 88.3% 84.3% 80.0% 476.0 Ponts Drift 97.8% 93.4% 92.2% 87.7% 86.3% 84.4% 79.9% 95/99% yield 35.0 Cumberland 128.3% 110.3% 107.1% 127.6% 128.0% 127.7% 127.4% 61.2 Buffelsdrift 110.6% 106.4% 105.3% 120.5% 122.5% 122.4% 123.8% 71.7 Riversdale 111.3% 105.6% 104.S% 104.2% 117.9% 117.5% 118.6% 100% yield 3S.0 Cumberland 131.0% 112.9% 106.9% 130.2% 131.4% 131.4% 130.2% 61.2 Buffelsdrift 112.2% 107.1% lOS.1% 119.4% 126.3% 126.6% 127.4% 71.7 Riversdale 112.9% 107.1% l04.S% 103.3% 119.9% 119.9% 119.6% The results in Table G6.7 exclude any assessment of the yields at higher reliabilities for the Lower Group of sites as these dams are not being considered to supply urban demand, to which such reliabilities apply. For the Upper Group of sites the higher Crocodile flows are instrumental in increasing the yields by between about 5% and 30%, with CumberIand the most significantly affected. The picture is not greatly changed by the adoption of different reliability criteria, although the increase is somewhat higher for the higher reliabilities. The Crocodile represents some 80% of the flow upstream of Cumberland and so the increase of 18% represents a general increase of about 15%; hence the variations in yield can be seen to be reasonably consistent with variations in flow. The effect on the Lower Group of sites is to reduce yield by up to 27%, although the yields for Ratho and Ponts Drift are generally only reduced by 15% of the base case. This is as a result of the greater losses which are assumed to occur along the river. The river-loss model, described in Annex F, Section FS.5, could only be G-60 calibrated for the reach of the river upstream of the gauging station. There has been no way of quantifying the apparently different morphology of the middle and lower reaches of the river which may be indicative of lower rates of loss. If this factor were to be proven it is most likely that flow sequences closer to the base case would result. The exercise was repeated for the second alternative flow series, with the results presented in Table G6.8. Alternative Flow Series 2 is based on the assumption that 40% of unaccounted losses occur in the target area. In order to achieve this further reductions were made to the Crocodile (down a further 5%) and Lephalala flows (down a further 12%). CumberIand is again the most affected of the upstream sites, although the average reduction in yield of about 5% is consistent with the reduction in inflow, with yields for Buffelsdrift and Riversdale hardly affected. Table G6.8 Comparison of Yields for Alternative Flow Series 2 Yield as a proportion of base case yield Chainage Dam site Reservoir storage (m 3106) (km) 250 500 750 1000 1 250 1 500 2000 95% yield 35.0 Cumberland 92.9% 97.3% 96.9% 96.2% 95.5% 95.1% 95.0% 61.2 Buffelsdrift 97.9% 98.0% 98.3% 95.4% 96.2% 95.1% 96.0% 71.7 Riversdale 99.1% 100.0% 98.3% 97.4% 97.1% 96.9% 95.8% 451.3 Mopani 98.5% 101.0% 100.6% 103.3% 108.2% 116.7% 118.5% 470.8 Ratho 100.1% 99.9% 100.2% 101.7% 103.1% 106.5% 106.6% 476.0 Ponts Drift 100.1% 100.1% 101.2% 101.6% 103.3% 105.5% 107.2% 80% yield 35.0 CumberIand 97.2% 96.4% 94.0% 96.9% 97.2% 96.7% 95.8% 61.2 B uffelsdrift 97.7% 97.7% 97.1% 98.2% 96.9% 97.7% 97.0% 71.7 Riversdale 98.4% 99.0% 97.5% ' 96.3% 98.5% 98.8% 97.1% 451.3 Mopani 100.2% 100.0% 103.3% 107.1% 106.9% 109.0% 116.2% 470.8 Ratho 100.9% 100.9% 101.9% 102.8% 106.7% 106.2% 110.0% 476.0 Ponts Drift 101.6% 101.1% 102.1% 104.3% 104.9% 106.7% 110.7% 95/99% yield 35.0 Cumberland 93.5% 97.3% 98.0% 92.8% 92.9% 92.3% 92.4% 61.2 Buffelsdrift 97.6% 97.7% 98.2% 94.1% 94.3% 93.4% 93.9% 71.7 Riversdale 97.2% 97.8% 98.5% 95.8% 95.3% 95.4% 95.0% 100% yield 35.0 Cumberland 89.7% 98.0% 98.4% 91.6% 92.8% 92.4% 91.6% 61.2 Buffelsdrift 95.1% 97.4% 98.4% 93.4% 93.4% 93.7% 94.3% 71.7 Riversdale 95.0% 98.0% 98.7% 97.7% 94.8% 94.8% 94.6% G-61 For the Lower Group of sites the 50% reduction in river losses (compared with the base case) can be seen to offset the reduction in the Crocodile and Lephalala inflows, but the differences are small. In general tenns this assessment of yield variation as a result of alternative hydrological assumptions has shown that differences are generally small, although in accordance with the magnitude and direction of the adjustment employed to derive the alternative flow sequences. The differences must be considered within the context of the overall hydrological studies, which are heavily reliant on the Oxenham/Sterkloop gauging record. As reported in Annex E: Calibration of Gauging Weirs, it was not possible to fully resolve apparent anomalies between the Oxenham Ranch and Sterkloop records at higher flood levels: also the nature of the measuring weirs introduces further areas of doubt. These factors would suggest that a tolerance of at least 5% must be placed on the flow records. Added to this are the subsequent processes of naturalisation/denaturalisation and the catchment modelling exercise whereby the flow record was extended. The yield analysis also introduces further areas of approximation. Whilst the digital terrain models of the reservoir basins produced for this study are substantially better than the infonnation obtained from the 1:50000 published maps, the accuracy cannot be expected to be as great as would be obtained from larger scale ortho-photo mapping based on specially commissioned aerial photography. Other factors such as inaccuracies in estimating evaporation and, to a lesser extent, rainfall must also be taken into account. It is, therefore, concluded that the different assumptions with regard to the basic hydrology result in variations in yield that are within the general accuracy of the yield estimation process. Accordingly, it is proposed ~at the economic sensitivity to variation in yield be assessed on the basis of a general variation of ±20%. In applying this blanket sensitivity factor it is important not to loose sight of the fact that a greater degree of reliance can be placed on the inflow sequences for the Middle Group of sites, by virtue of their proximity to the Oxenham/Sterkloop gauging stations, although this cannot be quantified. G6.4 THE EXISTING SYSTEM G6.4.1 Yields from the Existing System When assessing the merits of a dam on the Limpopo the yield of the existing system must be taken into account. Section F3.5 of Annex F sets out the assumptions used in deriving a series of "dummy dams" to represent the 73 weirs that have been located along the Upper Limpopo. These dummy dams have been used to estimate the current yield of the system. For the purpose of the hydrological studies the main stem of the Limpopo was divided into seven modelling units, each of which contained a dummy dam. The location of the modelling units is given in Figure F1.1 in Annex F. The "demand" on these darns was set by the estimated amount of irrigation in each modelling unit for the 1981, when irrigated areas were at the maximum; the details are summarised in Table G6.9. No allowance has been made for Talana Farm, in unit M070 on the Botswana bank, as this is assumed to draw its water from the Motloutse. G-62 Table G6.9 Existing Demand on the Upper Limpopo Modelling Chainage Irrigated area (ha) Demand unit (km) (m31Q6) Botswana RSA Total MO 10 0-60.8 0 2 103 2103 27.4 M020 60.8 - 151.7 1 152 1048 2200 27.7 M030 151.7 - 285.0 706 2402 3 108 40.3 M040 285.0 - 306.8 314 2796 3 110 39.8 M050 306.8 - 349.5 323 4734 5057 63.8 M060 349.5 - 469.0 135 2107 2242 27.2 M070 469.0 - 512.3 0 1 521 1 521 18.5 Total 2630 16711 19341 244.7 The system response to these demands is recorded in Table G6.1O. Table G6.10 Yields from the Existing System Modelling unit Total MO 10 M020 M030 M040 M050 M060 M070 Inflow 257.8 278.8 276.2 444.5 418.0 452.9 417.2 Demand 27.4 27.7 40.3 39.8 63.8 27.2 18.5 244.7 Average yield 9.6 11.6 11.7 15.7 19.5 16.8 12.2 97.1 Shortfall 17.8 16.2 28.5 24.1 44.2 10.3 6.4 147.5 Evaporation 0.5 1.1 1.0 0.8 0.6 1.1 0.5 5.6 Spillage 247.7 2662 263.5 428.0 397.8 434.9 404.6 Average storage 0.7 0.8 0.5 0.7 0.6 0.9 0.1 Maximum storage 2.3 2.5 2.3 1.9 2.0 2.2 0.3 70% yield 3.3 7.4 4.5 9.4 7.0 13.8 11.2 56.6 80% yield 0.4 5.5 1.5 2.9 1.6 10.4 9.6 31.9 From Table G6.10 it can be seen that there is a 60% shortfall in meeting the assumed demand. This clearly demonstrates that the current river flow pattern and existing weirs are unable to support the level of irrigation that has been assumed, a fact that is confmned by the significantly reduced areas currently under irrigation. Table G6.1O also provides an indication of what yields may be available at 70% and 80% levels of reliability. These figures show the poor performance of the small and shallow storages afforded by the farm weirs. G-63 A number of factors should be considered when assessing the results given in Table G6.1O. The pattern of demand on the dummy dams (see Section F4.4 in Annex F) is based on an assumption of 150% cropping intensity (full summer and half winter season cropped area). If farmers were limited to one crop then the reliability of supply would increase. The poor response of the dummy dams to the assumed yield pattern tends to confIrm the perception that even 50% winter cropping is not generally possible. It must also be appreciated that a constant demand pattern has been used throughout the 64 year simulation. Given the variability of the flow regime of the Limpopo it is clear that the fru:mers must respond to periods of high and low flow, as and when they occur, and make adjustments to their cropping patterns on a season-by-season basis: this will again improve the "reliability" of the yield. A fmal factor would be the possibility of conjunctive use of groundwater with supplies from the weirs. This is of crucial importance for some farms (eg Seleka Farm in Botswana), although the full extent could only be assessed by considering each individual farm. As a result of the initial assessment of the yields from the existing system it was concluded that the assumed demands were considerably in excess of what the farmers are able to extract on a long-term basis. In order to attempt to quantify this factor, the full analysis was repeated with the demand from the dummy dams reduced to the level of the average yield in the previous configuration: the results are presented in Table G6.11. Table G6.11 Yields from the Existing System with Reduced Demand Modelling unit Total MOlO M020 M030 M040 M050 M060 M070 Inflow 257.8 283.1 283.6 455.7 436.6 479.8 435.2 Demand 9.6 11.6 11.7 15.7 19.5 16.8 12.2 97.1 Average yield 4.7 7.0 5.4 7.5 8.7 12.8 9.2 55.3 Shortfall 4.9 4.7 6.3 8.1 10.8 4.0 3.0 41.8 Evaporation 0.7 1.5 1.5 1.0 0.9 1.4 0.6 7.6 Spillage 252.4 274.7 276.7 447.2 427.0 465.6 425.5 Average storage 0.9 1.0 0.8 0.8 0.7 1.2 0.2 70% yield 2.8 5.4 3.6 5.7 4.9 11.8 8.3 42.5 80% yield 0.8 4.9 1.5 2.5 3.0 9.9 7.8 30.4 From the table it can be seen that reducing the demand on the system also leads to a reduction in average yield, although the shortfall is reduced to about 40%. Whilst the factors noted above could play a significant role it would appear likely that even the demand figures assumed for this second assessment are probably an overestimate of the quantity relied upon by the farmers. From the preceding discussion it is clear that an accurate assessment of the current yield of the system would require considerably greater sophistication in analysis technique and substantially more data on individual farmers' cropping patterns and strategies for exploiting this unpredictable resource. Such an approach is not appropriate for the current level of study. For the purpose of the current exercise the "yield" of the current system has been taken as the values given in Table G6.1O, although this is may be a considerable overestimate. G-64 G6.4.2 Effect of Dam Construction Having obtained an estimate of current yield it is then necessary to establish how the current system will be affected by the construction of a dam. At any given dam location the flow immediately downstream is substantially reduced, although there will be some spills. Working progressively down the river the impact of the dam diminishes as a result of flows from incoming tributaries. Also, the quantity of flow originating at an upstream point is, in any case, reduced by losses down the river. To investigate this complex interaction the river model was alternately configured with dams at CumberIand, Buffelsdrift. Sunnyside and Selika. The spills from the upper two sites were based on reservoir storages of 750 m31 Table G6.12 Reduction in Yield from Existing Weirs as a Result of Dam Construction Chainage Dam site Reduction in average yield with dam in place (m3106) (km) Modelling unit Total M020 M030 M040 M050 M060 M070 35.0 Cumberland 1.5 2.2 0.8 1.0 0.0 0.0 5.5 61.2 B uffelsdrift 7.5 3.3 1.4 1.9 0.3 0.1 14.5 271.7 Sunnyside - - 3.3 4.6 0.7 0.1 8.7 290.8 Selika - - - 11.9 0.9 0.2 13.0 Of particular interest is the difference between the impact of a dam at Selika, compared with one at Sunnyside. The results from the initial yield studies, summarised in Table G6.4, had shown the difference in yield between Sunnyside and Selika to be surprisingly small, given the contribution' of the Lephalala (MAR 98.7 m310~ w~ich inflows downstream of the Sunnyside dam site (see Figure G 1.1). It was apparent that the poor storage characteristics of the Selika site were instrumental in negating much of the benefit expected from flows enhanced by the contribution of the Lephalala. A dam at Sunnyside offers the potential of the continued use of the Lephalala flows by the farmers along the Limpopo. The final analysis performed in relation to the yield from the dummy ~ams was to attempt to quantify the impact of progressive development of the water resources of the upstream tributaries. As discussed in Chapter F6 of Annex F. the hydrological sequences have been generated for two different time-horizons. along with the 1990 development level series, which constitutes the base case. The results, including the difference in yield when compared with the base case, are presented in Table G6.13. G-65 Table G6.13 Yield from Existing Weirs for Alternative Time-horizons Development Modelling unit Total level MO 10 M020 M030 M040 M050 M060 M070 1980 Development level Inflow 308.9 328.2 320.0 509.9 480.0 507.6 444.9 Demand 27.4 27.7 40.3 39.8 63.8 27.2 18.5 244.7 Average yield 12.6 12.9 13.1 16.9 22.2 17.2 12.3 107.2 Evaporation 0.6 1.3 1.1 0.9 0.7 1.2 0.5 6.3 Spillage 295.7 314.1 305.8 492.1 457.0 489.2 432.1 Average storage 0.9 0.9 0.6 0.8 0.6 1.0 0.2 70% yield 8.7 8.6 6.0 10.4 9.1 14.4 11.4 68.6 80% yield . 4.2 5.5 2.3 5.4 4.9 11.4 9.9 43.6 Difference in yield compared with base case 3.0 1.3 lA 1.2 2.7 0.4 0.1 10.1 Future development level Inflow 214.8 243.7 244.9 364.5 342.3 388.0 308.4 Demand 27.4 27.7 40.3 39.8 63.8 27.2 18.5 244.7 Average yield 5.2 10.5 10.2 14.3 14.4 16.4 11.0 82.0 Evaporation 0.3 0.9 0.8 0.6 0.5 1.1 004 4.6 Spillage 209.3 232.2 233.9 349.6 327.4 370.6 297.0 Average storage 0.5 0.7 0.5 0.5 0.4 0.9 0.1 70% yield 0.3 7.0 2.8 6.3 2.2 13.4 9.5 41.5 80% yield· 0.1 5.0 1.5 4.4 0.9 lOA 8.3 30.6 Difference in yield compared with base case -4.4 -1.1 -1.5 -lA -5.1 -004 -1.2 -15.1 G6.43 Compensation Requirements Having established the reduction in yield as a result of dam construction it is then necessary to estimate the . required releases in order to maintain the current levels of use. As the effect of the dam is registered at a considerable distance downstream, conveyance losses associated with any compensation releases must also be considered. The hydrological studies, reported in Annex F, divided river losses into three components, namely evaporation from the weir pools, river and bank losses and the "black-box" loss (see Section F5.5). These have been established on a monthly basis for each of the modelling units. As the compensation releases are intended to reproduce the current situation it has been assumed that releases would be made in two months of the year, with the existing weirs providing the necessary storage to complete the cropping cycle. Such a release pattern was seen as involving lower losses than an alternative pattern of continuous release. The resulting gross G-66 Dummy Dam Yields Downstream of a Limpopo Dam 20 · ,, " ...... ,,~"""" .. · , ,- ... - ...... · - --...... · , 15 ...... ••.•.....•...... t...... •...... ~ ...... •...... ~ ~-;~~:':·/r~····;·r·-··::-·················· i i ", /: / : : ", :/ i i ,/' / i: 1-----: .... ------:--: .. ." " " " I : b' ... .,. I I, I . -' tJ 10 g ••••••••••••••• ..·-1: .. :~:.~~·~·~·····f· ...... ~ ...... ··~·J(:.~················t+·············7-··· ··················f··································· ... . "0 r~· :,' I . : 8 ~ ' : , s :;:: , : ,. ... ' I '< I' ••~. , ,'" ! : tJ ' ,... : I : ' ... I I I "",,": I : S 5 ..... l...... ~ ...... ;_t"..~. ~.: •••••••••••.••••.••• ~ ••.••••.••..••.•••.•.••••••••• '1""'.' ...... ! ...... ~ (t) " i.' • i I i ~ I 0.. ~' i : en 'I ,::':: : ' , I I • ' , ' ::I I :I tJ I , :: : 0 ' , I I I ' I I I ~ ' ' , '::I I : :::l o ': I en..... (a 100 200 300 400 500 o ~ Chainage (km) 8 0 Legend >-t, Dam atCumberland ~ Dam at Buffelsdrift ~ Dam at Sunnyside §. Dam at Selika "0 0 No dam "0 'T1 o 0;;' tJ@ 0 0- S ~ compensation requirements for each dam site are given in Table G6.14, split between Botswana and RSA. In assessing the requirements for compensation it was assumed that no releases would be made to any modelling unit for which the yield was reduced by less than 5%. Table G6.14 Compensation Releases Compensation releases (ml 106) Dam site RSA Botswana Total Cumberland 10.2 2.5 12.7 Buffelsdrift 12.8 6.2 19.0 Riversdale 12.2 5.4 17.6 Sunnyside 10.2 1.1 11.3 Selika 29.4 2.5 31.9 Martins Drift 21.3 1.6 22.9 Worcester 15.4 1.0 16.4 Graaf Reinet '9.4 0.6 10.0 G6.5 YIELDS FROM A MULTI-USE DAM The yield analysis reported in Section G6.3 is based on the dam being operated to meet a single demand reliability. In practice the dam will probably be required to simultaneously supply urban demand, at a 95/99% reliability and irrigation demand at an 80% reliability. In order to establish the quantities available for such an arrangement it is necessary to simulate reservoir operation to establish two trigger levels on a trial and error basis: the first trigger level dictating the cessation of irrigation supplies, the second to reduce urban supply to 80% of the target. The first arrangement was for an equal share of the water between Botswana and RSA, with the entire RSA portion devoted to irrigation. The first call on the Botswana portion is taken to be the compensation releases, given in Table G6.14, with the remainder available for meeting urban demand. As a first step the 80% yield from any site was assumed to be equivalent to the 95/99% yield. Treating Selika at 1 000 m3106 as an example, the 80% yield is 173.6 m3106/yr, whereas the 95/99% is 60.1 m1106/yr (see Table G6.4) resulting in 1 m3 for urban supply being equivalent to 2.9 m1 for irrigation. This ratio varied from 7.8 (Cumberland at 250 m31Q6) to 2.3 (Graaf Reinet at 1 500 m3106), with a pronounced tendency for the ratio to be higher for smaller reservoir sizes. Using this "conversion" factor the required compensation yields for Botswana farmers were expressed as an equivalent yield at 95/99%. In this way the "target" urban supply was set as half the full 95/99% yield, less the compensation allowance. Reservoir simulations were then undertaken to establish how much could be supplied at an 80% reliability, whilst meeting the target urban demand. It was expected that the amount available for irrigation would be equivalent to half of the 80% reliability yield, plus the additional releases required as compensation flow for Botswana irrigation. The actual yields were G-67 significantly less than expected, generally between 65% and 90%. This shows that operating the reservoir to supply at one reliability tends to interfere with the potential for delivery of supplies at a different reliability. Hence, it was concluded that the target yield set for urban demand represented a greater than 50% share to Botswana. Revised target yields were then established by reducing the original urban demand by half the shortfall in irrigation supplies. expressed as a proportion. Again taking the example of Selika reservoir at 1 000 m31Q6: the shortfall in irrigation supply was 32%, hence the target urban demand was reduced by 16% to 19.1 m3 106/yr. The yield analysis was then repeated, with results that were much closer to the revised expected yield for irri·gation. A third round of adjustment was required for a number of the reservoir capacities. The results of this analysis are presented in Table G6.15: these yields constitute "Case I" as considered in the economic analysis, reported in Annex J. A number of different splits in demand between urban and irrigation were then investigated. Case 2 represents an alternative development for Botswana, whereby an allowance is made to increase the supplies to irrigation beyond those required for compensation. To investigate the significance of this approach the urban yields derived for Case 1 were reduced by 30% and the resulting increase in the availability of irrigation supplies was established - the results are given in Table G6.16. which constitutes Case 2 for the economic analysis. Case 3 is taken as all of Botswana's share being devoted to increased irrigation development, with overall yields as given in Table G6.4 for the 80% reliability criterion. Case 4 is based on a 75{25% sharing of the water, to take greater advantage of the higher economic benefits in meeting urban demand. Accordingly the amount going to meet urban demand was set as 50% greater than the Case 1 yield and the reservoir simulation exercise repeated to establish the yield available for irrigation - the results are presented in Table G6.17. Presenting yields as a set of numbers in a table does not provide any appreciation of the actual operation of the reservoir. As an aid to gaining such an appreciation Figures G6.5 to G6.8 depict the operation of Cumberland, Buffelsdrift, Sunnyside and Selika reservoirs for the Case 1 yields, as given in Table G6.15. The three sites in the upstream group are set to a storage capacity of 750 m3106, with the two from the middle group at 1 000 m31~. The annual summary of reservoir operation under the conditions covered by the figures is given in Appendix G-B. G-68 BOO ------700 600 500 400 300 E -u ::lE 200 :;0 -- 100 (D ID (/) E (D ...... :::J 0 =2 0 o > -.""1 800 (j) C- •.-1 0 S' > 700 e...... C- ...... ~ m 600 ...... (I) o m ::l 0: 500 I (/) ~ 400 I \!\j ." ""1 n \ c: 300 " I \ S '\ eT 200 (D "'\1 :::.. 100 5 0.. 0 ...... >;.:l -J VI 'Tj o (Jc,-. Legend Cumber land dam at 750 Mcm: Yields 10.1 Mcm (95/99%) and 37.1 Mcm (80%) - FSl and dead storage levels Sw ~ ._._-- ChDsen tr11lOor levels ...... 0 MONTHLY RESERVOIR VOLUMES 00 ~ u, BOO 700 600 500 400 300 -e u 200 ~ ?:1 - 100 ('P w C/J e ('P ...... ::J 0 ~ 0 o > ...,-. c- BOO Cl) 'H 0 s· > 700 s c- ...... p.) w 600 ...... (f) o (1) ;:l c: 500 C/J o..., 400 .', '""" to 300 :;:~ ...... ('P 200 C/J 0.. ...,...... 100 ::P p.) 0 ...... -J Ul 'T1 o (J«-. Legend Buffelsdrift at 750 Mcm: Yields 13.6 Mcm (95/99%) and 52.1 Mcm (BO%) S", @ -- FSL and dead storage levels p..-i. Cl ._-_ .. ,. Chosen trigger levels o 0 MONTHLY RESERVOIR VOLUMES 0, 0\ 1000 900 BOO 700 600 500 400 e -u aoo e200 ;:0 (!) ID tOO CI'l E (!) ~ ...... ::J 0 o 0 ...... > ""'I 1000 CI:l ...... c.. §' 0 900 c:: > ...... c.. BOO ...... ~ ID ..... en o Ql 100 ~ CC CI'l 600 ~ 500 ""'I CI:lc: 400 S 300 '< ...... CI'l 200 0.. (!) 100 ...... ~ 0 ...... 1!1fl o o 'T1 o oe;-, Legend Sunnyside at 1'000 Mcm: Yields 24.3 Mcm (95/99%) and 65.4 Mcm (80%) 8 t: -- FSL and dead storage levels u.J @ "--_•• ChoSIlIl tr1goer levels ...... 0 ' 1_-.... o 0 MONTHLY RESERVOIR VOLUMES 0'> I -J ------.--, 1000 900 BOO \ 700 ~\ 600 500 \J\ 400 E300 -u ~2OO (]) 100 E ~ (1) ::l 0 CIl ...... (1) 0 > ~ 1000 o...... c... "1 '8 900 en > c... BOO ~. (]) en ...... (]) 700 ...... ~ Cl: o· 600 ::l CIl 500 0- 400 "1 Cl) (1) 300 ...... 200 ::-;-' ~ 100 ...... p;l ...... 0 o o '11 o (JQ-. Legend Selika at 1 000 Mcm: Yields 24.5 Mcm (95/99%) and 74.5 Mcm (80%) a,-, Si('p - FSt. and dead storage levels ...... 0 [' ---.. Chosen trigger levels 00 MONTHLY RESERVOIR VOLUMES a, co Table G6.15 Joint Yields for 50/50% Share Yield at stated reliability (m3J06/yr) Dam site Reservoir storage (m3J06) 250 500 750 1 000 1 250 1 500 2000 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% Cl I 0\ '0 Cumberland 15.7 1.5 26.3 5.3 37.1 10.1 41.3 12.2 45.5 12.6 53.9 13.4 59.0 13.7 Buffelsdrift 34.9 4.8 40.1 8.2 52.1 13.6 62.0 17.6 65.0 17.3 78.1 19.6 81.4 18.9 Riversdale 34.3 5.6 48.9 11.3 54.7 16.2 67.1 23.6 73.9 24.5 76.3 24.1 94.2 26.6 Sunnyside 23.2 5.2 40.7 10.8 56.5 18.5 65.4 24.3 80.7 31.7 81.1 35.6 99.1 35.3 Selika 39.2 6.5 49.6 11.6 60.6 18.1 74.5 24.5 83.3 28.0 93.9 32.7 114.5 39.5 Martins Drift 40.1 7.2 52.4 11.6 58.9 18.4 73.4 24.7 81.4 27.8 89.1 33.0 109.4 39.2 Worcester 40.9 9.2 55.4 15.9 63.0 22.6 81.1 29.7 87.8 34.9 100.0 39.3 117.3 47.0 Graaf Reinet 27.9 7.6 46.9 17.0 62.8 25.4 75.8 30.8 90.5 37.8 103.0 42.3 129.9 48.2 L....-_____ ...... Table G6.16 Joint Yields with 30% of notswana's Share Allocated to Irrigation , Yield at stated reliability (m3 106/yr) Dam site Reservoir storage (m3 106) 250 500 750 1 000 1 250 1 500 2000 o 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% SO% 95/99% ~ o Cumberland IS.0 1.0 31.1 3.7 43.6 7.0 55.7 8.6 56.4 8.S 62.6 9.4 71.6 9.6 Buffelsdrift 39.5 3.4 4S.7 5.8 63.S 9.5 73.5 12.3 79.8 12.1 77.S 13.7 106.7 13.2 Riversdale 40.8 3.9 53.5 7.9 66.4 11.4 81.5 16.5 91.9 17.1 94.2 16.S 114.0 18.6 " Sunnyside 27.7 3.6 4S.3 7.6 71.2 12.9 77.S 17.0 99.0 22.2 109.3 24.9 121.5 24.7 Selika 39.8 4.5 59.7 8.1 68.3 12.7 90.8 17.2 102.2 19.6 108.6 22.9 136.9 27.6 Martins Drift 40.7 5.0 61.0 8.1 67.7 12.9 90.8 17.3 10l.6 19.5 110.4 23.1 131.2 27.4 Worcester 47.0 6.4 66.0 11.1 76.4 15.8 96.7 20.8 108.9 24.4 121.4 27.5 142.7 32.9 Graaf Reinet 33.1 5.3 56.0 11.9 78.0 17.S 100.1 21.6 109.7 26.5 129.9 29.6 160.3 33.8 ------~----.-- Table G6.17 Joint Yields with 75% to Urban Demand in Botswana Yield at stated reliability (m 3 106/yr) Dam site Reservoir storage (m3106) 250 500 750 1 000 1 250 1 500 2000 a 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% I -...l ..... Cumberland 12.3 2.2 18.6 7.9 25.6 15.1 28.1 18.3 27.9 18.9 32.6 20.1 34.0 20.6 Buffelsdrift 26.2 7.2 27.3 12.3 34.5 20.3 38.3 26.5 41.1 26.0 41.5 29.4 48.9 28.3 i I Riversdale 21.6 8.4 29.4 16.9 36.0 24.3 41.3 35.3 44.2 36.7 47.9 36.1 53.5 39.9 - Sunnyside 16.1 7.8 29.3 16.2 33.6 27.7 42.2 36.5 40.9 47.5 43.6 53.4 55.0 53.0 Selika 19.9 9.7 36.6 17.4 44.5 27.2 46.3 36.8 54.9 42.1 59.7 49.0 69.0 59.2 Martins Drift 16.6 10.8 37.2 17.5 44.5 27.6 45.3 37.1 53.4 41.7 55.1 49.5 64.5 58.8 Worcester 22.4 13.8 38.6 23.8 46.9 33.8 50.7 44.6 53.8 52.4 61.2 58.9 66.1 70.4 Graaf Reinet 19.4 11.4 34.7 25.5 42.3 38.1 48.4 46.2 52.7 56.7 64.4 63.5 73.7 72.4 ... I ...... I .... G6.6 YIELDS FOR A MULTI-DAM DEVELOPMENT The development of a dam from the Upstream Group would stilI leave significant resources untapped. leaving open the possibility of a further development at one of the Middle or Lower Group of sites. To investigate the potential for such an approach. yields have been derived for the Middle and Lower Group sites with a dam at Cumberland or Buffelsdrift, sized at 750 m31Q6. Spills are included based on the dams being operated to supply a single 95% reliability yield, representing 47.2 and 57.3 m31Q6/yr for Cumberland and Buffelsdrift, respectively. The resulting MAR's for each dam site are presented in Table G6.18, along with those for the future development scenario which is covered in Section G6.8. Table G6.18 Comparison of MAR's with Different Development Options MAR (m31Q6) Dam site Base case Dam at Dam at Dam at Future Cumberland Buffelsdrift Selika development Cumberland 230.1 - - - 195.1 Buffelsdrift 254.4 - - - 209.7 Riversdale 275.2 - - - 230.4. Sunnyside 340.9 249.2 244.9 - 288.2 Selika 412.1 325.2 320.8 - 331.6 Martins Drift 412.1 325.2 320.8 - 331.6 Worcester 412.1 325.2 320.8 - 331.6 Graaf Reinet 365.3 288.2 283.2 - 296.0 Mopani 287.5 266.7 252.4 209.7 221.3 Ratho 403.5 382.7 368.4 325.7 292.7 Ponts Drift 403.5 382.7 368.4 325.7 292.7 In establishing these yields consideration has only been given to the 80% reliability criterion (all sites) and the 95199% reliability criterion (Middle Group of sites). The results are presented in Tables G6.19 and G6.20. As a final exercise in this series the multi-yield potential of the sites was investigated, with Buffelsdrift in place, on the basis of a 50/50% split between Botswana and RSA, following the procedure outlined in Section G6.5; the results are presented in Table G6.21. The 64 year record of volume fluctuations for a Selika dam (at 1 000 m3106) operated to give the results given in the table is depicted in Figure G6.9; again the equivalent annual results summary is provided in Appendix G-B. The potential yields from a development of one site from the Lower Group, with a Selika dam in place (at 1 000 m3 1Q6) have also been established, for an 80% yield criterion: the results are presented in Table G6.22. G-72 1000 900 800 700 ~ ::0 I ~ 600 ! C/) ~ 500 ~ I o 400 I ..... ~ {\~ >-i E3OI) \J C/:l -u I \ ~200 S· ! -, ,. s Q) 100 ------I· 1 ~ E ...... ::I 0 § ...... C/) 0 > 6 1000 >-i '- .;; 900 C/:l ~ > ...... '-800 ~ Q) i ~ enQ) 700 ~ ~ 0: 600 (\,/ ...... i o 500 g 400 ~~ 300 \ ...... 200 ~ ~ 100 & 0 0:1 c>-h '11_. >-+) oc; ::: Legend ~...... Selika at 1 000 Mcm with Buffelsdrift: Yields 13.4 (95/99%) and 59.6 (80%) C/) (l) - - FSl. and dead storage levels p... 0 -,,-,_ .. Chosen trigger levels ::1. eJ'.. MONTHLY RESERVOIR VOLUMES ;:::r \.0 Table G6.19 Yields with a Dam at Cumberland Yield (m3106/yr) Chainage Dam site Reservoir storage (m3106) (km) 250 500 750 1000 1250 1500 2000 80% reliability 138.7 Sunnyside 45.8 70.0 92.9 107.7 117.7 131.8 144.3 290.8 Selika 68.9 95.8 120.6 143.2 157.2 169.1 191.8 304.2 Martins Drift 69.7 95.7 121.3 142.6 155.6 165.8 187.9 317.7 Worcester 75.8 104.6 129.9 153.1 167.9 179.7 200.6 337.0 Graaf Reinet 59.6 91.2 117.9 145.1 159.2 175.0 198.9 451.3 Mopani 84.6 127.0 157.1 175.7 196.3 216.8 235.6 470.8 Ratho 162.1 213.5 250.7 279.9 303.8 317.2 352.8 476.0 Ponts Drift 157.6 203.4 241.3 271.6 293.0 310.0 339.4 95/99% reliability 138.7 Sunnyside 9.3 14.2 20.2 23.8 23.8 23.8 23.8 290.8 Selika 17.9 23.0 27.7 33.3 38.1 38.0 38.0 304.2 Martins Drift 19.2 23.9 28.5 33.7 37.8 37.8 37.7 317.7 Worcester 23.9 30.0 35.8 42.2 47.3 47.3 47.2 337.0 Graaf Reinet 16.6 23.3 29.9 37.4 41.2 41.2 41.1 Table G6.20 Yields with a pam at Buffelsdrift Yield (m3IQ6/yr) Chainage Dam site Reservoir storage (m3IQ6) (km) 250 500 750 1000 1250 1500 2000 80% reliability 138.7 Sunnyside 37.3 61.0 83.3 93.3 105.5 115.7 131.6 290.8 Selika 61.5 86.8 107.8 127.3 141.1 152.7 174.9 304.2 Martins Drift 62.3 87.0 107.2 . 126.1 137.5 148.9 169.4 317.7 Worcester 66.8 95.0 116.0 137.6 151.0 162.5 184.0 337.0 Graaf Reinet 52.9 82.4 109.7 130.5 149.7 161.3 187.6 451.3 Mopani 73.4 118.8 144.4 163.7 185.7 203.8 220.6 470.8 Ratho 157.6 203.8 245.2 273.2 291.4 311.8 337.9 476.0 Poots Drift 151.1 199.7 235.5 262.6 282.3 300.9 325.8 95/99% reliability 138.7 Sunnyside 6.4 10.2 15.7 19.4 19.4 19.4 19.4 290.8 Selika 13.8 21.3 25.6 29.9 34.3 34.4 34.9 304.2 Martins Drift 14.8 22.3 26.2 30.2 34.6 34.5 34.6 317.7 Worcester 19.5 27.8 32.7 37.7 42.4 42.7 43.2 337.0 Graaf Reinet 13.1 19.4 26.5 34.1 39.0 39.3 39.2 G-73 Table G6.21 Joint Yields with a Dam at Buffelsdrift for a 50/50% Share Yield at stated reliability (m3106/yr) Dam site 3 6 o Reservoir storage (m 10 ) ~ 250 500 750 1000 1 250 1500 2000 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% 80% 95/99% Sunnyside 16.9 2.7 26.0 4.3 59.1 9.5 57.4 9.9 --. 37.3 6.5 43.6 8.8 50.9 9.1 Selika 26.5 5.4 38.9 8.8 51.6 11.4 59.6 13.4 68.8 14.9 73.2 15.6 87.4 16.0 Martins Drift 27.1 6.1 38.3 9.6 51.2 12.0 59.1 13.8 65.2 16.0 71.6 16.1 76.1 15.9 Worcester 29.3 8.4 43.2 12.4 56.7 15.5 66.3 17.8 73.6 20.4 77.3 20.7 87.3 20.5 Graaf Reinet 20.5 5.2 35.4 8.6 48.2 .11.5 59.3 14.9 65.9 17.5 80.3 19.2 104.7 21.3 ------Table G6.22 Yields with a Dam at Selika Yield (m3106/yr) Chainage Dam site Reservoir storage (m 31Q6) (km) 250 500 750 1 000 1 250 1 500 2000 80% reliability 451.3 Mopani 62.3 90.7 113.6 134.9 150.1 162.2 166.5 470.8 Ratho 144.7 182.7 217.5 238.8 259.6 276.9 293.1 476.0 Ponts Drift 137.4 176.2 208.1 230.6 249.7 267.3 282.6 G6.7 SHAPANE OFFSTREAM STORAGE An offstream storage site, Shapane, was identified in the course of the Dams Studies, see Section D7.7 of Annex D. The site offers the potential of reduced evaporation by virtue of improved storage characteristics. Such a development would rely on a comparatively large pumping capacity from a Limpopo dam to transfer water to the Shapane site. Two darns are within a practical distance of the Shapane site, Selika and. Martins Drift, but in view of the high cost of a Martins Drift dam (see Annex D, Table DS.l), the assessment has been limited to the Selika Dam site. In assessing the potential of such a development there are three parameters to be considered, namely the storage capacity of Selika and Shapane, and the pumping capacity between the two. The higher reliability criterion used for urban supplies results in water remaining in the reservoir for long periods in order to bridge the extended periods of drought: these circumstances offer the greatest potential for benefit from reduced evaporation as a result of transfer to the offstream storage site. In order to simulate the joint operation of the Limpopo dam in this case a demand has been placed on the darn equivalent to twice the transfer capacity to Shapane. No account is taken of reliability, the transfer is made whenever water is available. The transfers thus generated are then treated as inflow to the Shapane site, after being halved to represent a 50% allocation to Botswana: the other 50% represents the RSA portion of reservoir yield. The 95/99% reliable yield is then established using conventional reservoir simulation for the Shapane site. At this preliminary level no account has been taken of the requirement for a portion of Botswana's share to be allocated to meeting compensation releases for irrigation, nor the interference effect from the multi-use operation that would be necessary, as discussed in Section G6.5. Both these factors would tend to reduce the yield from the system. The results are presented in Table G6.23, with the example of Selika at 750 m3 1Q6 presented graphically in Figure G6.10. G-7S Table G6.23 Yields of Shapane together with Selika 95/99% yield (m3IQ6/yr) Pumping rate (m3IQ6/yr) 25 37 50 75 100 150 200 Selika at 250 m3106 Pumping time 88% 81% 76% 65% 57% 47% 38% Shapane volume 125 12.7 15.6 18.1 17.9 16.7 15.4 14.5 (m310~ 250 12.6 15.6 18.1 22.3 24.7 26.6 26.0 500 12.6 15.5 18.1 22.2 24.9 29.3 32.1 750 - - - -- 29.3 32.3 1000 ------32.4 Selika at 500 m3106 Pumping time 94% 88% 82% 75% 68% 57% 48% Shapane volume 125 17.6 21.1 23.6 24.1 . 21.0 18.1 16.8 (m3106) 250 17.6 21.0 23.6 28.4 31.9 31.9 29.9 500 17.6 21.0 23.6 28.3 32.1 37.7 39.2 750 - - - - 32.0 37.8 39.5 1000 ------32.4 Selika at 750 m3106 Pumping time 98% 92% 87% 79% 73% 64% 54% Shapane volume 125 22.6 26.0 28.6 29.7 24.3 20.4 17.2 (m310~ 250 22.5 26.0 28.6 32.6 36.3 35.8 35.8 500 22.5 26.0 28.6 32.6 36.5 43.2 43.2 750 - - - - - 43.4 43.4 1000 ------ Selika at 1 000 m3106 Pumping time 100% 95% 90% 82% , 76% 67% 57% Shapane volume 125 24.4 30.3 32.3 34.8 26.6 20.6 17.8 (m31O~ 250 24.4 30.3 32.2 35.6 39.8 38.2 32.2 500 - - - 35.6 39.9 46.9 48.6 750 - - - - - 47.0 49.7 1000 ------49.7 G-76 95/99% Yields of Shapane with Selika Selika dam at a storage of750 Mcrn 50 -:....---- I It, ~ , • 40 _~ ___ - ______- ______.. ______• ______~_ ------. ______~------.------.------~ ------:,;..f • .r::'___ ------~------0------~ ------: j 1 _ ~ 1 l , ·! .,...... ,,-~------~------~------~------. , : .,.,,- ,.,-: : ' ,.,...... l--. -- .i .i • .,....,.. ,,-- :I :, :, , , ~ 30 >\ ______• ______- • ____ M ______--- ______.1,.----- ____ M ______" ______0. ______~ __ :;:: 20 · , >-'~ • (D >-' 10 0... 7 ------(/J 0 >-+, C/) ::r C-l 'D o 5 o 25 50 75 ](X) 125 150 175 2(X) (D ..-t- Pumping capacity (m' IO"/yr) 0 (Tq Legend ,..,.(D ::r Shapane at 125 Mcrn (D Shapane at 250 Mcrn >-1 Shapane at 500 Mcrn ~ ...... 'T1_. 0"< ::r c C/) 2 ~C; r,.>-. C\ f-j 0 As a fmal test, the influence of increasing the Botswana proportion of the yield to 75% was assessed (equivalent to Case 4 in Section G6.5). This was only considered for one reservoir size at Selika, with the results presented in Table G6.24. Table G6.24 Yields of Shapane together with Selika with 75% of Total Yield to Urban Demand in Botswana Selika at 750 m3106 Pumping rate 150 (m 3106/yr) Pumping time 73% Shapane volume 95;99% yield (m3 1 125 26.1 250 48.0 500 60.0 750 60.0 G6.8 EXPECTED YIELDS FOR THE FUTURE DEVELOPMENT SCENARIO The hydrological sequences used to establish the yields from the existing system under future conditions (the year 2020) have also been used to assess the likely future yields of the Limpopo dam sites. The resulting MAR at each site is in Table G6.18. The assessment has been limited to the 95/99% yield criterion for the Upper and Middle Group of sites and the 80% criterion for all the sites. The results are presented in Table G6.25, as a proportion of the base case yields given in Table G6.4. From the table it may be seen that there is a general reduction in yield to about 50% to 75% of the base case. The average yield for the Upper Group of sites is reduced to 69%. The average for the Middle Group of sites is reduced to 60%, with the impact of reduced flows in the Lephalala producing a proportionately bigger reduction in the yields of the lower four in the group when compared with the results for Sunnyside. The average yields for the Lower Group of sites is reduced to 65%. It is interesting to note that the reduction is, in general terms, fairly consistent down the river, indicating that the potential developments are reasonably equally spread throughout the catchment area. G-77 Table G6.2S Comparison of Yields for Future Development Flow Series Yield as a proportion of base case yield Chainage Dam site Reservoir storage (m 3106) (km) 250 500 750 1000 1250 1500 2000 80% reliability 35.1 Cumberland 57.0% 62.0% 66.2% 71.0% 71.1% 71.1% 73.5% 61.2 Buffelsdrift 63.1% 64.4% 64.9% 68.0% 69.4% 70.5% 72.6% 71.7 Riversdale 67.0% 71.9% 71.7% 71.0% 74.6% 75.4% 74.8% 138.7 Sunnyside 70.4% 71.5% 75.0% 76.1% 74.7% 76.6% 75.8% 290.8 Selika 46.0% 53.9% 59.6% 61.7% 64.8% 67.2% 67.1% 304.2 Martins Drift 46.7% 53.5% 59.5% 61.6% 63.9% 66.4% 67.6% 317.7 Worcester 49.0% 56.6% 61.8% 64.4% 67.0% 68.1% 68.7% 337.0 Graaf Reinet 50.4% 62.2% 66.7% 67.8% 72.2% 72.4% 73.4% 451.3 Mopani 61.9% 68.2% 70.1% 70.2% 74.1% 73.1% 72.2% 470.8 Ratho 57.3% 59.5% 61.9% 62.2% 65.4% 65.1% 66.7% 476.0 Ponts Drift 55.4% 59.4% 61.8% 61.6% 64.0% 64.4% 66.1% 95/99% reliability 35.1 Cumberland 51.5% 68.2% 68.6% 71.6% 72.2% 71.4% 71.3% 61.2 B uffelsdrift 56.3% 71.6% 73.0% 65.2% 65.0% 64.2% 64.3% 71.7 Riversdale 59.7% 75.9% 81.4% 71.8% 71.9% 71.5% 71.0% 138.7 Sunnyside 45.8% 56.9% 64.8% 71.4% 69.2% 64.1% 63.2% 290.8 Selika 55.1% 52.6% 54.3% 55.2% 57.7% 56.0% 50.4% 304.2 Martins Drift 57.0% 54.8% 55.1% 56.6% 58.0% 56.2% 52.0% 317.7 Worcester 58.2% 56.1% 56.6% 579% 58.8% 59.1% 54.4% 337.0 Groaf Reinet 47.5% 52.6% 53.7% 57.4% 60.0% 55.8% 55.2% G6.9 RELIABILITY OF YIELDS IN THE EARLY YEARS The analysis undertaken previously establishes the long-term yield. The period immediately after construction is a special case in the analysis of yields, particularly in the case of the Limpopo where the flow is very unreliable. With regard to meeting urban demand if there was a probability that the required yield would not be available in a particular year then it would be necessary to plan construction of the dam to allow a period for dam impounding. The characteristics of the reliability of yield from a particular site are dependent on a number of factors, namely, reservoir capacity, target yields, trigger levels, etc. Each set of circumstances constitutes a separate case, requiring detailed analysis, which is considerably more complex than the basic yield analysis. Due to time constraints it has not been possible to analyse all 11 sites, at each of 7 reservoir capacities, for all of [he potential cases considered in the economic analysis. Accordingly a selection of sites has been assessed, generally at a single reservoir capacity. G-78 The results of the preliminary economic analysis suggested the selection of the following dam sites and reservoir capacities: • Cumberland at 750 m3106 • Buffelsdrift at 750 m31~ • Riversdale at 750 m3 IQ6 • Riversdale at 1 000 m3106 • Sunnyside at 1 000 m3106 • Selika at 1 000 m3IQ6 The reliability of supply in the flrst ten years after commissioning has been derived by assuming that the dam is constructed in every possible year of the record; hence, for the 64 year flow sequence each dam is "commissioned" in each year from 1924 to 1977. The reliability of supply in any "year one", for example, is based on the number of non-failure months in each of the 54 "year ones", divided by the total number of months. It is assumed that the dam starts impounding in October in any particular year (the start of the hydrological year) but no demand is placed on the dam until the following January. The yield that the dam is expected to provide is based on the "Case I" Scenario, as presented in Table G6.15. However, the urban demand in the early years will not be the full potential yield, but rather the net requirements of the system. This is taken to be 3.7 m3 1Q6 in 1998, 5.7 m 3IQ6 in 1999, etc: for a discussion on the projected demands see Chapter J5 in Annex J. For each particular site the urban demand is capped at the theoretical long term yield, although as the yield in 2007 is 20.3 m31Q6 the capping does not come into effect in the ten year period for Selika., Sunnyside and Riversdale at 1 000 m3106• The irrigation demand is set to the theoretical long term yield in all years, except for the fIrst year of supply, when 50% is set as the target. Trigger levels are employed in the same way and at the same level as for the steady-state analysis. To investigate the effect of allowing various periods for reservoir impoundment a varying period has been allowed when no yield abstractions are made, and no allowance made for compensation releases. A sample output from this analysis is given in Appendix G-B. The results of these analyses are presented in Tables G6.26 to G6.31, in terms of the reliability of yield in the three categories of demand, urban, reduced urban (80% of urban) and irrigation, with target reliabilities of 95%, 99% and 80% respectively. From Table G6.26 it can be seen that general unreliability of flow has a more pronounced effect on the Cumberland site, where even allowing a four year grace period does not achieve the required reliability for meeting urban demand. In subsequent years the reliabilities begin to increase, with that for irrigation exceeding the target. More detailed analysis would be require.d to ascertain if any improvements could be achieved as a result of modifications to the trigger rules, or a more gradual increase in the releases for irrigation. However, such considerations can also be applied to the other sites and, therefore, the results presented in Table G6.26 to G6.30 can be treated as a comparative assessment. A further point to consider relates to the practicalities of allowing no releases for irrigation for a prolonged period. It is also likely that some releases will be required for compensation to existing farmers during the period allowed for impounding, although this should not be necessary in years of zero inflow, as compensation releases would not be intended to improve on the current situation. Nevertheless, any compenSation releases will further reduce the reliability for meeting urban demand. In conclusion, the issue of the reliability of supply in the early years of operation of a Cumberland dam is considered to cast grave doubt as to the feasibility of such a development. G-79 Table G6.26 Reliability of Meeting Demand with a Dam at Cumberland Storing 750 m310' Years Demand Reliability of meeting demand (% of months) allowed for Year after corrunissioning dam impounding 1 2 3 4 5 6 7 8 9 10 0 Urban 57.7 71.5 79.2 83.5 88.4 89.4 92.7 93.7 95.7 96.6 Reduced urban 57.9 74.7 80.2 85.2 91.0 92.3 94.8 96.0 97.2 98.9 Irrigation 53.2 57.9 64.4 70.2 74.7 77.6 80.2 81.9 83.2 85.0 1 Urban - 75.6 79.6 84.6 90.3 90.6 93.4 93.4 95.2 96.0 Reduced urban - 75.9 83.0 86.1 91.2 94.1 94.8 96.9 97.1 98.5 Irrigation - 68.1 68.1 71.6 75.8 78.4 80.9 83.2 83.8 85.0 2 Urban - - 83.6 86.0 90.9 92.4 92.9 94.4 95.7 97.2 Reduced urban -- 84.7 87.8 91.2 94.6 96.0 96.1 97.7 99.1 Irrigation -- 76.5 74.7 77.8 80.1 80.9 83.6 84.3 85.0 3 Urban -- - 89.7 91.4 94.4 94.4 95.7 94.9 97.1 Reduced urban --- 90.0 91.4 94.8 96.5 98.5 98.0 98.3 Irrigation - - - 80.1 79.0 80.9 82.7 86.3 86.0 85.5 4 Urban ---- 93.1 94.8 96.3 96.9 97.2 98.0 Reduced urban -- - - 93.1 94.8 96.6 98.5 98.8 99.7 Irrigation - --- 80.6 82.6 84.6 86.4 88.3 87.8 Note: No compensation releases allowed for during filling period Table G6.27 Reliability of Meeting Demand with a Dam at Buffelsdrift Storing 750 m310' Years Demand Reliability of meeting demand (% of months) allowed for Year after corrunissioning dam impounding 1 2 3 4 5 6 7 8 9 10 0 Urban 78.5 86.7 91.2 93.2 92.4 92.1 94.3 93.5 95.5 96.6 Reduced urban 79.2 88.1 91.2 93.4 95.4 94.8 97.1 98.3 98.5 100.0 Irrigation 62.3 62.3 65.7 72.7 77.0 79.0 82.1 82.6 84.0 85.2 1 Urban - 89.5 91.7 93.1 94.6 93.5 95.5 96.3 96.1 97.5 Reduced urban - 91.0 93.7 94.9 96.5 96.0 97.2 98.5 99.4 100.0 Irrigation - 71.8 66.4 73.9 79.2 80.2 81.5 81.9 84.0 85.2 2 Urban - - 92.7 93.2 94.3 95.2 96.9 96.9 96.3 98.1 Reduced urban - - 96.3 95.8 98.5 98.1 97.4 98.5 99.1 100.0 Irrigation - - 76.4 76.7 81.0 81.5 82.1 83.2 84.6 85.2 3 Urban - - - 96.5 96.3 95.1 96.5 98.1 98.9 99.2 Reduced urban -- - 98.3 99.5 99.8 99.4 98.6 100.0 100.0 Irrigation - - - 81.0 81.9 82.6 82.7 85.5 85.6 85.2 4 Urban - - - - 98.0 97.2 96.6 98.3 97.8 99.1 Reduced urban - - - - 100.0 100.0 99.8 99.4 100.0 100.0 Irrigation - - - - 84.7 84.6 86.4 88.3 88.1 86.7 Note: No compensatIOn releases allowed for during filling period G-80 The position at Buffelsdrift, given in Table G6.27, is not as bad as Cumberland, with reliabilities close to their targets allowing for a three year impounding period, a situation that is maintained throughout the period covered. The requirement for irrigation compensation releases from Buffelsdrift is more severe (Table G6.14). Similar arguments would also apply to the need to adjust trigger levels and allow for a slower build-up in irrigation demand. However, such detailed assessment can only be undertaken in Stage II of the study. For the purpose of the pre-feasibility assessment' it is considered that a three year impounding period should be allowed for B uffelsdrift. Two reservoir sizes have been considered at Riversdale. Even though a larger reservoir may provide a higher long-term yield it will not necessarily provide any greater security of supply in the short-term. As shown in Tables G6.28 and G6.29 the reliability of supply from the smaller dam is slightly higher than for the larger dam in the early years, although this is probably mainly due to the lower irrigation demand (65.4 m3IQ6/yr compared with 74.5 m3106/yr for a reservoir capacity of 1 000 m3 IQ6). The table suggests that a two year allowance for impounding may be adequate for the 750 m3 IQ6 reservoir, on the assumption that better trigger rules could be established to allow reduced reliability of the irrigation supplies to permit a greater reliability for meeting urban demand. It should be noted, however, that the reliabilities in year 4 for a case of a two year allowance for impounding are below the target, suggesting that further reduction in irrigation releases may be desirable. For the situation with Riversdale at 1 000 m3 1Q6 the results suggest that a three year impoundment period would be required, although the possibility of reducing the irrigation supplies would appear to offer the potential of reproducing the circumstances of the smaller reservoir size. It is therefore considered that a two year impounding period should be allowed for the Riversdale site: further consideration is given to this point in Chapter 14 of the Main Report. Table G6.30 presents the results for Sunnyside, which can be seen to be only marginally better the Cumberland site. It is recommended that a four year period be allowed for Sunnyside. The results for Selika are presented in Table G6.31: these represent the best of the dams considered. It would appear that a two year fllling period should be allowed for this dam, with some additional curbs on irrigation releases in the early years. The foregoing analysis has revealed differences between the sites with regard to the reliability of supply in the early years, which must be seen as one of the crucial issues for the feasibility of a dam on the Limpopo. Unfortunately this type of analysis is much more complex than the basic assessment of long-term yields and it has only been possible to consider in general terms at this stage of the study. Stage II studies will be required to look into this aspect further. In particular the issue of minimum releases for irrigation during the period of impoundment will have to be considered, along with the allowable rate of growth in irrigation releases through the critical early years. This latter point raises the issue of what is considered to be a 50/50% share of the water in these early years. G-81 Table G6.28 Reliability of Meeting Demand with a Dam at Riversdale Storing 750 m310' Years Demand Reliability of meeting demand (% of months) allowed for Year after commissioning dam impounding 1 2 3 4 5 6 7 8 9 10 0 Urban 78.9 85.8 90.3 91.0 94.0 92.9 92.0 92.6 94.0 95.7 Reduced urban 79.5 87.7 90.6 93.4 94.9 95.5 97.2 98.1 98.3 100.0 Irrigation 61.7 63.6 69.1 74.4 81.6 83.0 84.6 85.6 85.6 86.1 1 Urban - 88.7 91.4 94.0 94.8 93.8 93.7 92.6 94.4 96.6 Reduced urban - 89.0 91.7 94.3 95.7 95.7 97.2 98.5 99.4 99.8 Irrigation - 75.0 73.5 78.1 82.3 83.5 84.7 86.9 86.3 86.9 2 Urban -- 94.9 94.1 96.0 95.4 96.0 97.1 94.8 96.5 Reduced urban - - 95.4 94.3 96.5 96.1 97.2 98.5 100.0 100.0 Irrigation -- 83.5 80.6 82.9 84.0 85.2 87.7 88.1 87.5 3 Urban - -- 96.9 96.3 96.1 97.4 98.1 98.1 98.0 Reduced urban --- 96.9 96.6 96.6 97.4 98.5 100.0 100.0 Irrigation - - - 91.2 85.8 85.6 86.6 88.3 88.9 89.8 4 Urban - - - - 98.6 97.2 97.4 98.5 98.9 99.5 Reduced urban - - - - 98.9 97.4 97.5 98.5 100.0 100.0 Irrigation - - - - 94.0 89.2 89.0 89.4 90.6 92.0 Note: No compensation releases allowed for during filling period Table G6.29 Reliability of Meeting Demand with a Dam at Riversdale Storing 1 000 m310' Years Demand Reliability of meeting demand (% of months) allowed for impounding Year after commissioning dam 1 2 3 4 5 6 7 8 9 10 0 Urban 78.7 85.8 89.0 91.2 94.1 93.2 95.1 95.7 96.5 97.8 Reduced urban 79.2 87.8 89.4 92.4 94.8 95.7 97.2 98.5 99.1 100.0 Irrigation 60.6 62.5 67.0 73.1 77.3 78.2 82.1 84.3 85.8 86.7 1 Urban - 89.8 91.0 92.6 93.5 93.8 94.6 96.5 97.5 97.7 Reduced urban - 91.0 92.9 94.6 96.1 95.7 97.4 98.5 100.0 100.0 Irrigation - 71.0 68.4 75.0 80.7 82.1 83.2 85.0 86.9 88.3 2 Urban - - 93.2 92.4 94.3 94.1 95.4 97.1 98.0 97.2 Reduced urban - - 94.6 96.0 98.1 97.4 97.2 98.5 100.0 100.0 Irrigation - - 77.8 79.0 82.7 82.6 82.9 87.0 88.9 89.4 3 Urban - - - 95.4 93.5 94.6 96.5 98.3 98.3 97.8 Reduced urban - - - 97.4 98.3 99.5 98.6 98.5 100.0 100.0 Irrigation - - - 84.6 83.0 83.8 85.0 87.7 88.9 90.1 4 Urban - - - - 95.5 94.9 96.5 98.3 99.7 100.0 Reduced urban - - - - 98.3 100.0 99.5 99.1 100.0 100.0 , Irrigation - - - - 88.9 87.5 87.0 88.3 89.4 90.6 Note: No compensatIOn releases allowed for dW'lIlg filling period G-82 Table G6.30 Reliability of Meeting Demand with a Dam at Sunnyside Storing 1 000 m310' Years Demand Reliability of meeting demand (% of months) allowed for Year after commissioning dam impounding 1 2 3 4 5 6 7 8 9 10 0 Urban 64.8 79.5 85.0 88.3 93.1 95.1 94.8 97.2 97.2 96.5 Reduced urban 64.8 79.6 85.2 88.3 93.1 97.4 97.1 98.5 100.0 99.8 Irrigation 58.0 64.4 70.7 75.3 80.7 83.6 84.6 86.4 87.0 88.3 1 Urban - 82.1 86.0 89.4 92.9 96.6 95.7 98.1 98.9 97.5 Reduced urban - 83.8 87.7 90.9 93.2 96.6 97.4 98.5 100.0 100.0 Irrigation - 74.7 70.7 75.8 80.9 83.6 85.0 86.4 87.5 88.3 2 Urban - - 86.4 89.2 92.7 97.4 97.4 98.1 99.1 98.5 Reduced urban - - 92.4 95.1 95.2 98.5 100.0 100.0 100.0 100.0 Irrigation -- 77.8 78.4 80.7 84.4 85.6 86.4 88.3 88.9 3 Urban --- 91.4 92.4 98.0 97.8 98.0 98.3 99.5 Reduced urban - - - 96.3 96.6 98.5 100.0 100.0 100.0 100.0 Irrigation - -- 83.3 83.5 85.8 88.7 88.1 88.3 88.9 4 Urban - --- 94.6 98.0 98.5 98.3 100.0 98.8 Reduced urban --- - 98.1 100.0 100.0 100.0 100.0 100.0 Irrigation --- - 86.4 87.5 89.5 90.7 90.0 88.9 Note: No compensatlOn releases allowed for dunng fillmg penod Table G6.31 Reliability of Meeting Demand with a Dam at SeIika Storing 1 000 m310' Years Demand Reliability of meeting demand (% of months) allowed for Year after commissioning dam impounding 1 2 3 4 5 6 7 8 9 10 0 Urban 74.2 91.4 88.9 90.1 94.9 95.8 98.1 98.8 96.8 94.9 Reduced urban 74.7 91.7 93.2 96.1 98.1 100.0 100.0 100.0 100.0 99.2 Irrigation 65.9 72.1 74.5 76.4 80.7 84.1 85.8 87.0 87.0 87.5 1 Urban - 92.1 93.1 93.1 95.5 96.6 97.1 98.9 96.8 95.4 Reduced urban - 92.3 93.2 96.1 98.1 100.0 100.0 100.0 100.0 99.5 Irrigation - 80.7 78.2 80.4 82.7 85.5 86.4 87.0 87.5 88.3 2 Urban - - 95.5 96.1 97.8 99.4 97.4 96.5 98.8 95.8 Reduced urban - - 95.7 96.1 98.1 100.0 100.0 100.0 100.0 100.0 Irrigation - - 84.6 81.9 84.0 87.7 88.1 87.0 88.3 88.9 3 Urban - - - 98.0 98.1 100.0 99.8 99.2 98.5 97.8 Reduced urban - - - 98.1 98.1 100.0 100.0 100.0 100.0 100.0 Irrigation - - - 85.2 85.5 87.7 89.4 88.1 88.3 88.9 4 Urban - - - - 98.6 100.0 100.0 100.0 100.0 98.3 Reduced urban - - - - 98.8 100.0 100.0 100.0 100.0 100.0 Irrigation - - - - 88.9 89.8 90.1 90.4 88.9 88.9 Note: No compensation releases allowed for during filling period 0-83 G-84 CHAPTER G7 IMPACT ON WATER RESOURCES DOWNSTREAM OF THE STUDY AREA G7.1 IMPACT ON FLOWS AT THE END OF THE STUDY AREA Flow sequences have been developed for a point at the end of the study area, just upstream of the Shashe confluence, reflecting different development options. The background to the methods employed in deriving the flows is given in Chapter F6 of Annex F. The results are summarised in Table G7.1 as MAR's and as a proportion of the base case MAR. Table G7.1 MAR Upstream of the Shashe Confluence Flow sequence MAR Proportion of (m3106) base case Naturalised 707.3 193% Base case (current development level) 367.1 100% Alternative flow series 1 279.4 76% Alternative flow series 2 432.8 118% Dam at Cumberland: capacity 750 m3 106 348.4 95% Dam at Buffelsdrift: capacity 750 m3106 334.6 91% Dam at Sunnyside: capacity 1 000 m31Q6 312.3 85% Dam at Selika: capacity 1 000 m3 IQ6 295.1 80% Future development scenario 266.8 73% 1980 development level 392.9 107% A comparison between MAR's for the naturalised state and the base case hydrology shows that the impacts of existing developments have substantially reduced flows at the end of the study area, and the future impacts are small in relation to the changes that have taken place in the past. The significance of the alternative hydrological sequences is brought out in the table. As discussed in Section G6.3.2 the different characteristics of the lower portion of the river, below the Sterkloop gauging station, would suggest that losses through this reach would be lower. Hence, even if it were established that the high river losses associated with Alternative Flow Series 1 were appropriate for the upstream section of the river it is unlikely that the same rate of loss would apply downstream; consequently the MAR upstream of the Shashe confluence is unlikely to be as low as 279.4 m3106• However, the only way of firmly establishing the position at the lower end of the study area would be to install a flow measuring station. Given the possible implications of not having accurate estimates at this location on future discussions with the other countries in the Limpopo Basin it is recommended that consideration be given to the installation of a flow recording station. G-85 The table also brings out the relative impact of a Limpopo darn compared with the possible scale of general development of the water resources of the catchment, which will result in a marked reduction in flow. On the other hand it can be seen that the impact of developments in the decade of the eighties has not been very si gnifican t. It is recognised that the MAR is not necessarily a good measure of the availability of useful water, due to the distorting effect of the occasional large flows that cannot be harnessed in any practical way. Accordingly a comparison has been made of the yields from Ratho darn site, adopting an 80% reliability criterion and a storage capacity of 1 000 m31Q6: the results are presented in Table G7.2. Table G7.2 Yields from Ratho at a Storage of 1 000 m310' Flow sequence Yield at Proportion of Yield as 80% reliability base case proportion of (m3106/yr) MAR" Base case 307.6 100% 84% Alternative flow series 1 269.9 88% 97% Alternative flow series 2 316.3 103% 73% Dam at Cumberland 279.9 91% 80% Dam at B uffelsdrift 273.2 89% 82% Dam at Selika 238.8 78% 81% Future development scenario 192.8 63% 72% 1980 development level 333.2 108% 85% * MAR at end of study are as given in Table G7.1 The table shows that the variation in MAR is the predominant factor in dictating yields in this downstream reach. However, the degree of development in the upstream catchment can also be seen to affect the proportion of the total flow that can be practically utilised, in particular the reduction in flow associated with the future development scenario is compounded by a reduction in the yield as a proportion of flow. G7.2 ADDITIONAL WATER RESOURCES DOWNSTREAM OF THE STUDY AREA G7.2.1 Shasbe River Three sources of information have been consulted to obtain estimates of flow in the Shashe river. BNW:MPS derived estimates of flow for the Lower Shashedam site (BNWMPS, Volume 6), located just upstream of the Ramaqabane tributary, which constitutes the border with Zimbabwe. Estimates for the Zimbabwe tributaries have been taken from a note prepared by the Zimbabwe Ministry of Energy and Water Resources and Development, dated 9 March 1990. Estimates for the runoff from the Botswana portion of the catchment below the Lower G-86 Shashe dam site, covering some 192 km of river, have been taken from L \VUS, where it was designated as "Shashe Riparian". The MAR's for the tributaries and the main stem of the Shashe river are given in Table G7.3. Table G7.3 MAR for Shashe River Tributary/location Indicative MAR (m3106) Lower Shashe dam gauging site 125 Ramaqabane 42 Sansukwe 13 Semakwe 51 Shashani 64 Tuli 281 Hwali 11 Shashe riparian 10 Total at Limpopo confluence 597 The MAR given for Lower Shashe excludes the catchment of the existing Shashe Dam (BNWMPS, Volume 6, Table 5.1) and may therefore be considered as denaturalised. Of potential significance is the possible development of Lower Shashe dam, although there are no published estimates of spillage. The figures for the Zimbabwe tributaries appear to refer to the naturalised flow set and no detailed estimates of current development levels are readily available. The tributaries of the Limpopo constitute Hydrological Zone B in the Zimbabwean classification system. The zone has a total MAR of 1 157 m3 106 with an estimated level of annual use of 213 m3106 (J\1MP, Hidroprojecto, 1990a). A number of potential developments have been identified, a possible 400 m3 106 capacity dam on the Tuli (ibid) being the most significant. It is not known how far advanced are the plans for such developments. The MAR at the existing Shashe darn site is 92 m3106, hence the total naturalised MAR of the Shashe is about 689 m3 106, although it is not known whether any channel loss effects will also be a feature of this river. It would appear, therefore, that the contribution from the Shashe river was very similar to that of the Upper Limpopo in the natural state (see Table G7.1). G7.2.2 Along the Zimbabwe/RSA Border Contributions from Zimbabwean portion of the catchment draining into the 216 km of tl"!e Limpopo from the Shashe confluence to the Mozambique border are registered as five drainage basins. Estimates of MAR have been obtained from the same source quoted in the previous section. Three main tributaries drain the RSA portion. G-87 Estimates of MAR have been obtained from HRU (1981) for the Sand River and Nzhelele, whilst information on the Levuhu has been obtained from the Levuhu Basin Study (HKS, 1991) with an addition for the Mutali which joins close to the confluence with the Limpopo; the information is summarised in Table G7.4. Table G7.4 MAR for Tributaries between Shashe Confluence and the Mozambique Border Tributary Naturalised Current MAR (m3IQ6) development Mutshilashokwe (Zimbabwe) 12 - Umzingwani (Zimbabwe) 350 - Sand River (RSA) 75 40' Nzhelele (RSA) 76 40' Shabili (Zimbabwe) 23 - Babi (Zimbabwe) 53 - Pesu (Zimbabwe) 1 - Levuhu (RSA) 571 491 Total 1 149 - • MSC estimate No estimates were readily available for the current development level flows for the Zimbabwean tributaries. There is a flow gauging station at Beit Bridge on the stretch of the Limpopo between the Umzingwani and the Sand River confluences. A number of problems have been experienced in attempting to interpret this record (which runs from 1955 to 1980), but the best estimate of historic MAR is 1 950 m3IQ6. This may be compared with a combined estimate of naturalised MAR's upstream of the site of 1 758 m3IQ6. Unfortunately it is not possible to resolve this apparent conflict within the resources available to this study but the comparison suggests that there is a possibility that the losses in the Upper Limpopo downstream of the Sterkloop gauge may have been overestimated. G7.2.3 Tributaries that Join in Mozambique One major tributary draining Zimbabwe joins the Limpopo inside Mozambique, the Mwenezi; unfortunately no indication of the MAR is readily available for this river. The major tributary for RSA is the Olifants (in Mozambique the Elephantes) with a naturalised MAR of 2 510 m3106 and a current development MAR of 1 568 m3IQ6. These figures were derived by combining estimates for Letaba (SRK, 1991), which joins almost at the border, with those for the rest of the Olifants (TPGP, 1991). A smaller tributary, the Shingwidzi, has an MAR of 78 m3106 (SRK, 1991). All values refer·to estimates at the border between RSA and Mozambique. G-88 The drainage basin of a further tributary, the Changane, lies entirely within Mozambique, although no estimates of flow are readily available for this river or the main stem of the Limpopo. G7.3 IMPACT ON THE DOWNSTREAM STATES The demand for water from the Limpopo in Zimbabwe is not large (see Annex A, Section A9.1) with the town of Beit Bridge representing the most critical demand point. Agricultural development in this region of Zimbabwe has been restricted by the remoteness and, in recent years, by security problems. Within RSA there are significant areas of irrigation, drawing supplies from the sand aquifer of the Limpopo river channel. Within Mozambique the main demand is for irrigation, currently estimated as 21 450 ha (requiring 538 m3106(yr) in the Limpopo basin, with proposals to increase this to 156 000 ha (MMP, Hidroprojecto, 1990b). Much of the projected increase is based on using supplies released from the Massingir dam on the Elephantes. Problems have already been experienced with low river flows and saline intrusion upstream from the mouth of the Limpopo. From the foregoing analysis it would appear that the construction of a Limpopo dam would have minimal effect on river flows downstream of the Shashe. Taking, for example, a dam at Selika, the resulting reduction in MAR is estimated to be about 72 m3106 (see Table G7.1), which would represent only about 4% of the historic flow record at Beit Bridge (see Section G7.2.3). It must be appreciated that the Beit Bridge record is not very reliable and the current development level MAR is bound to be significantly less then the historic average of flows, hence a figure of 10% reduction may be more realistic. The impact on water use downstream cannot be assessed without reference to the details of the method of extracting supplies. No major man-made storages have been constructed on the main stem of the river, so exploitation is currently reliant on a run-of-river approach, subject to the carry over effects from water stored in the alluvium of the river bed. This method of utilisation is particularly susceptible to increasing the periods of no flow in the river, to which a Selika dam would contribute. However, the more important conclusion to come from this brief assessment is that the Limpopo dam is only one element in a continuous process of exploitation of the water resources of the basin, and it appears that the impacts of future developments in the upstream tributaries are more significant than a single dam on the Limpopo. Given the existing level of water use and the proposed developments, it would appear that Mozambique is the most likely to suffer through this process, establishing the requirement for additional storage capacity in the lower reaches of the river. It is not clear whether this would be an essential requirement in order to meet the planning objectives of Mozambique in any case. It is clear, however, that any programme of compensation releases from a Selika dam could not possibly contribute to the alleviation of problems in Mozambique. 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The Department of Geological Survey, Botswana, unpublished report JDV/l/85, November 1985. De Vries J J 1985b Groundwater Prospects study of the Tuli Block Farms Buffelsdrift, Saaspost, Dikglatong and Dead Mule. - Report to Derek Brink Holdings, July 1985. Gibb 1986 Lobatse Water Supply Refurbishment. Pre-Investment Study, Vol I and lI, for Water Utilities Corporation, Botswana. Gibb 1989 Woodlands and Township boreholes, Lobatse area, for Water Utilities Corporation, Botswana. Gieske A, Selaolo E 1990 Groundwater recharge through the unsaturated zone of south-eastern & McMullan S Botswana: A study of chlorides and environmental isotopes. - IARS pub!, 191. Regionalization in Hydrology. International Symposium at Lublijana, Yugoslavia 1990, pp 33-44. GITEC 1983 Seleka Farms Development StUdy. Interim Report, Botswana Development Corporation. HKS 1991 Water Resources Planning of the Levuhu River Basin, Hill Kaplan Scott, 1991. Hobbs P J & 1983 A preliminary evaluation of the groundwater resources of the Upper Limpopo Esterhuyse C J River valley, North-West Transvaal, Report GH 3278, Directorate of Geohydrology, DWA (RSA). Hobbs P J & 1986 Groundwater resource evaluation of the lower Crocodile River valley, North Chipps R J Western Transvaal, Report GH 3443, Directorate of Geohydrology, DWA(RSA). HRU 1981 Surface Water Resources of South Africa, Volume 1, Middleton B J, Pitman W V, Midgley DC and Robertson R M, Hydrological Research Unit, Report No 10/81. IoH 1986 Ramotswa wellfield, South-east Botswana, Digital studies and storage estimates. Institute of Hydrology for Sir Alexander Gibb & Partners. Jansen H 1982 The geology of the Waterberg Basins in the Transvaal. Memoir 71, Geological Survey, Department of Mineral and Energy Affairs, RSA. G-93 REFERENCES (CONTINUED) KataiO 1989 The Lerala-Moeng Valley Ground Water Exploration Project; Ground Water Resources Assessment - MSc thesis. University College London. The Department of Water Affairs, Botswana. unpublished report, Summer 1989. Keller S 1986 Summary of the results of hydro geological investigations in the Waterberg Supergroup between Molepolole and Mochudi and assessment of the ground water potential. Department of Geological Survey, Botswana, Unpublished Report SK/2/86. Keller S - 1988 Assessment of the Groundwater Potential of the Waterberg Area East of Mochudi (South-east Botswana). Department of Geological Survey. Unpublished Report SEK/4/88. Kempster P L et al 1982 Summarised water quality criteria. Department of Water Affairs. RSA, Technical Report No 1R 108. Kempster P L & 1985 Proposed aesthetic/physical and inorganic drinking-water criteria for the Smith R Republic of South Africa. NIWR. CSIR Research Report No 628, Pretoria. Kempster P L & 1988 Water quality objectives for raw waters for potable use, Internal Report, Van Vliet HR Hydrological Research Institute, Department of Water Affairs, RSA. MacDonald 1987 Limpopo Water Utilisation Study. - Report to the Department of Water Affairs, Botswana. MacDonald & 1990a Sub Saharan Africa Hydrological Assessment (SADCC Countries), Country Hidroprojecto Report: Zimbabwe MacDonald & 1990b Sub Saharan Africa Hydrological Assessment (SADCC Countries), Country Hidroprojecto Report: Mozambique Mulder? 1971 Waterlewering uit die bedding van die Mogolorivier N. Tvl, Report GH 1510, Directorate Geohydrology. DWA (RSA). Neumann-Redlin 1984 The Palapye Groundwater Exploration Project (1983-1984), Volume I and IT. The Department of Geological Survey, Botswana, unpublished report CNR/6/84, Oct and Dec 1984. Neumann-Redlin 1982 Serowe Water Supply - Groundwater Resources Study. Final Report. Department-of Geological Survey, Botswana, Unpublished Report 1982. G-94 REFERENCES (CONTINUED) NordM 1985 A Study of the Major Sand Rivers of Botswana. Phase n. - Department of Water Affairs, Botswana. O'Connor DJ 1989 Seasonal and long term variations of dissolved solids in lakes and reservoirs. Journal of Environmental Engineering, ASCE, Vol 115, No 6, 1213 - 1234. O'Keeffe J H et al 1990 The effects of impoundments on the physicochemistry of two contrasting southern african river systems, in Regulated Rivers: Research and Management, Vol 5, 97 - 1l0. Orpen W R G & 1982 Availability of groundwater in the Northern Transvaal region centred on Fayazi M Alldays, Report GH 3243, Directorate Geohydrology, DW A (RSA). Pretorius 1991 Landowner - Personal communication. Rech W D 1970 Groundwater survey along the Palala, Mogol, MatIabas and Crocodile Rivers, Geological Survey Report GH 1530, Department of Mineral and Energy Affairs, RSA. Richards L A 1954 Diagnosis and improvement of saline and alkali soils, United States Department of Agriculture (USDA), Handbook Volume 60, 160pp. SABS 1984 Specification for water for domestic supplies. South African Bureau of Standards Specification No 241 - 1984. Schumann F W ? Grondwater in die ArgeYese gesteentes noord van die Witfonteinrant Rustenburg Transvaal, Report GH 2909, Directorate Hydrology, DW A (RSA). Selaolo E 1985 Evaluation of aquifer hydraulic parameters in Ramotswa, SE Botswana, MSc thesis, University of London. Selaolo E 1986 Refinement of Aquifer Storage Calculations in Ramotswa Well field. - Department of Geological Survey, Botswana, Unpublished Report. SGAB 1988 Serowe Groundwater Resources Evaluation Project. - Final Report, Department of Geological Survey, Botswana. Simonis J J 1988 Grondwaterkwaliteit van die Matlabasrivier omgewing Noord-Transvaal, Report GH 3605, Directorate Geohydrology, DWA (RSA). G-95 REFERENCES (CONTINUED) SIoots R R & 1990 Groundwater Recharge to a Fractured Aquifer in SE Botswana: Results of Wijnen M M a survey of the Molepolole East Wellfield. University of Botswana, Gaborone. SMEC, WLPU, 1990 Botswana National Water Master Plan Study. Snowy Mountains Engineering SGAB Corporation, WLPU Consultants and Swedish Geological International AB for DWA Botswana. Draft Volumes 4 - Environmental Aspects and 5 - Hydrogeology. SRK 1990 Water Resources Planning of the Mogalakwena River Basin, Steffen, Robertson & Kirsten Inc. SRK 1991 Water Resources Planning of the Letaba River Basin, Steffen, Robertson & Kirsten Inc. SS&O 1990 Crocodile River (Western Transvaal) Catchment Study. Stewart Sviridov and Oliver in co-operation with BKS Inc to the Department of Water Affairs, RSA. Report 2290: Water Quality Situation Analysis (Draft). Timje H 1987 Letlhakeng Groundwater Investigation Project, Phase 2, for the Department of Geological Survey, Unpublished Report HT/l/87, 1987. TPGP 1991 Water Resources Planning of the Olifants River Basin, Theron, Prinsloo, Grimsehl and Pullen, 1991. 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A Reconnaissance of the Major Sand Rivers, for Department of Water Affairs, Botswana. WHO 1984 International standards for drinking-water, World Health Organization, Geneva. WLPU 1985 Ramotswa Wellfield Pollution Study, Final Report, for Department of Water Affairs, Botswana. WLPU 1989 Ramotswa Wellfield. Operation Procedures and Resource Management Project, for Water Utilities Corporation, Botswana. G-97 WATER Q UA LlT V ANAl. VSIS T ECHNIQUES APPENDIX G-A WATER QUALITY ANALYSIS TECHNIQUES LIST OF CONTENTS Page No G-A.l Technique Used to Estimate the Export of Salts from Tributaries G-A-l G-A.2 Techniques to Detect Trends in Small Water Quality Data Sets G-A-l G-A.3 Technique to Patch TDS Data Records from Measured Conductivity Values G-A-3 G-AA Sodium Absorption Ratio G-A-S LIST OF TABLES Page No G-A.l Statistical Analysis of RSA Tributary Water Quality Data G-A-2 G-A.2 MAR and TDS Load Estimates G-A-3 G-A.3 ECrrDS Linear Regression Coefficients G-A-4 G-AA TDS/Discharge Non-linear Regression Coefficients G-A-4 G-A.S Non-linear Regression Coefficients Assumed forRivers in Botswana G-A-S G-A.6 Classification of Water Quality for Irrigation Purposes G-A-S LIST OF FIGURES Following Page No G-A.l TDS vs Discharge for Mogalakwena River G-A-2 G-A.2 Time Series: TDS for Mogalakwena River G-A-2 G-A.3 Classification of Irrigation Water G-A-6 APPENDIX G-A WATER QUALITY ANALYSIS TECHNIQUES G-A.l TECHNIQUE USED TO ESTIMATE THE EXPORT OF SALTS FROM TRIBUTARIES The mass export of TDS from a tributary is estimated using the following method: • Monthly runoff sequences for each tributary over a period of 64 years (1924 to 1987) were obtained from the application and calibration of the Pitman Runoff Model. The method used in hydrological modelling is described in Annex F: Hydrology. • From measured TDS and discharge data the regression relationship between TDS concentration and river discharge was determined, as shown in Figure G-A.l. Table G-AA shows the regression coefficients calculated for the monitoring stations on each tributary. • The monthly TDS load is calculated using the regression relationship in conjunction with runoff data, where the load is given as the product of runoff times concentration. • Monthly TDS loads are summed to determine an annual load. Table G-A.2 shows the mean annual TDS loads for each tributary. • To verify the method, the discharge and quality data set for the Mogalakwena River was used to calculate monthly and annual TDS loads from daily flow records and instantaneous water quality data, for a period of one year. A difference of ± 15% was recorded between methods using daily data and monthly time steps. A statistical analysis of the available water quality data for the RSA tributaries is given in Table G-A.l. G-A.2 TECHNIQUES TO DETECT TRENDS IN SMALL WATER QUALITY DATA SETS Trends in the water quality of a river can be caused by increasing loads of effluent or agricultural runoff entering the water course. Such discharges cause the concentration of the rivers to rise, and/or the ionic composition of the river water to change. Figure G-A.2 (upper section) shows a time series of the TDS concentration data for the Mogalakwena River at station A6HOO9. The lower portion of Figure G-A.2 shows a section of the time series enlarged to show the variation in TDS concentration and discharge for a period of 2 years. The variation in TDS concentration over a high and low flow period shows a distinct seasonal trend, with low TDS during high flow and high TDS during low flow - a characteristic common to all the rivers in the basin. As there is a strong relationship between the TDS concentration and discharge it is necessary to "decompose" the data set with regard to discharge. For simplicity, the data is "decomposed" by plotting the TDS concentration versus discharge on a double log plot (see Figure G-A.l). The plot of TDS concentration versus discharge is analyzed graphically by examining if over G-A-l TableG-A.l Statistical Analysis of RSA Tributary Water Quality Data Station AIH002 A2H037 A3H007 A4HOO4 A4H007 A4HOI0 A4ROOI A5H006 A5R002 A6HOO9 I A6R002 A7H004 River Upper Crocodile Marico Matlabas Mol::olo Limpopo Lephalala Mogalal::weoa Umpopo Notwane EC Min. 37.0 13.6 5.4 0.5 1.2 6.2 4.6 7.3 4.2 4.4 I1.S 10.0 P50 48.3 67.0 41.0 4.0 4.4 8.0 7.5 24.9 23.0 53.0 41.0 30.0 P90 52.0 78.0 77.0 10.0 6.2 14.6 14.S 35.9 46.0 95.0 97.0 50.0 Max. 55.7 87.0 84.0 86.0 16.5 18.4 22.0 69.0 116.0 150.0 160.0 88.0 pH Min. 7.0 6.7 5.3 4.4 4.0 5.9 4.7 6.1 3.4 4.0 6.4 6.3 P50 7.8 7.8 7.9 6.4 6.0 6.7 6.5 6.7 6.8 7.5 7.4 7.0 P90 8.3 8.6 8.3 7.1 6.6 7.3 7.4 7.1 7.7 8.0 7.9 7.5 Max. 8.6 8.9 8.7 8.5 7.3 8.6 7.S 7.4 8.3 8.S 8.6 8.0 IDS Min. 288.0 89.0 44.0 12.0 *23.0 39.0 37.0 *4S.0 33.0 *84.0 84.0 72.0 PSO 393.0 486.0 325.0 3S.0 33.0 58.0 65.0 161.0 154.0 364.0 262.0 200.0 P90 444.0 548.0 629.0 66.0 44.0 97.0 105.0 232.0 285.0 640.0 609.0 306.0 Max. 512.0 6OS.0 709.0 577.0 72.0 134.0 159.0 445.0 653.0 952.0 1007.0 734.0 Ca Min. 29.0 8.0 5.0 0.0 0.0 3.0 2.0 8.0 1.0 1.0 7.0 7.0 P50 51.0 37.0 29.0 2.0 1.0 5.0 5.0 13.0 12.0 29.0 19.0 20.0 P90 57.0 45.0 36.0 5.0 2.0 12.0 12.0 18.0 25.0 36.0 30.0 27.0 Max. 62.0 46.0 41.0 46.0 7.0 20.0 17.0 29.0 53.0 48.0 40.0 90.0 Mg Min. 25.0 4.0 7.0 0.0 0.0 2.0 1.0 3.0 1.0 1.0 3.0 3.0 P50 32.0 20.0 33.0 1.0 1.0 3.0 3.0 9.0 6.0 15.0 10.0 10.0 P90 35.0 23.0 77.0 3.0 1.0 4.0 4.0 14.0 14.0 29.0 23.0 16.0 Max. 38.0 64.0 89.0 19.0 3.0 5.0 9.0 18.0 38.0 43.0 38.0 48.0 K Min. 0.0 2.1 0.7 0.0 0.0 1.0 0.8 1.1 0.4 0.3 0.2 1.0 P50 0.5 9.5 1.4 0.5 0.4 1.5 1.1 2.1 1.1 4.1 3.1 2.8 P90 0.8 11.2 3.0 1.3 0.8 2.0 2.0 3.7 2.2 6.2 7.1 4.2 Max. 2.2 12.8 5.5 14.0 3.3 2.5 2.5 4.8 3.5 8.8 14.3 6.1 Na Min. 2.0 9.0 3.0 1.0 3.0 4.0 3.0 6.0 4.0 3.0 9.0 6.0 P50 3.0 66.0 12.0 3.0 5.0 6.0 6.0 9.0 21.0 56.0 44.0 23.0 P90 4.0 81.0 29.0 9.0 7.0 7.0 11.0 28.0 47.0 113.0 123.0 44.0 Max. 6.0 94.0 46.0 93.0 8.0 10.0 12.0 33.0 95.0 215.0 220.0 50.0 Min. 168.0 36.0 16.0 0.0 0.0 13.0 15.0 26.0 14.0 11.0 34.0 27.0 Total P50 232.0 163.0 186.0 15.0 8.0 24.0 28.0 37.0 57.0 128.0 95.0 73.0 alkalinity P90 260.0 187.0 337.0 24.0 12.0 45.0 51.0 82.0 123.0 185.0 148.0 98.0 Max. 307.0 309.0 395.0 85.0 23.0 66.0 76.0 105.0 220.0 251.0 263.0 375.0 Cl Min. 1.0 8.0 2.0 0.0 1.0 4.0 2.0 4.0 4.0 3.0 10.0 5.0 P50 6.0 76.0 7.0 4.0 8.0 6.0 5.0 24.0 27.0 79.0 63.0 28.0 P90 10.0 93.0 19.0 13.0 11.0 11.0 10.0 36.0 64.0 171.0 185.0 63.0 Max. 15.0 103.0 31.0 85.0 17.0 18.0 15.0 53.0 159.0 292.0 333.0 73.0 F Min. 0.0 0.2 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 P50 0.1 0.6 0.3 0.0 0.0 0.1 / 0.1 0.3 0.2 0.3 0.3 0.3 P90 0.2 0.7 0.4 0.2 0.1 0.3 0.1 0.5 0.4 0.6 0.5 0.5 Max. 0.3 1.0 0.8 0.9 0.3 0.5 0.2 0.9 0.6 1.1 0.7 1.0 Si Min. 5.4 0.8 2.3 1.0 2.0 1.8 2.1 2.3 3.1 1.5 3.3 1.0 P50 6.4 4.7 6.4 3.3 4.8 3.3 3.8 4.8 4.1 5.7 5.7 6.8 P90 6.9 6.8 8.0 4.3 5.6 4.3 4.9 6.3 5.0 7.7 8.0 8.3 Max. 15.8 22.0 9.0 6.4 8.8 7.1 8.9 7.6 5.8 11.2 8.7 15.0 S04 Min. 0.0 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 P50 5.0 66.0 20.0 2.0 2.0 4.0 4.0 12.0 5.0 12.0 7.0 13.0 P90 10.0 77.0 54.0 5.0 6.0 9.0 9.0 23.0 12.0 29.0 19.0 31.0 Max. 26.0 90.0 86.0 170.0 13.0 13.0 13.0 47.0 28.0 56.0 42.0 38.0 NH4 Min. 0.000 0.020 0.000 0.000 0.000 0.000 0.010 0.000 0.000 0.010 0.000 0.010 P50 0.040 0.050 0.030 0.040 0.040 0.050 0.040 0.020 0.030 0.040 0.030 0.030 P90 0.110 0.100 0.080 0.120 0.110 0.100 0.070 0.060 0.060 0.120 0.140 0.080 Max. 0.430 0.160 0.150 MOO 0.210 0.290 0.160 0.200 0.100 0.290 0.230 0.090 N03 Min. 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 P50 1.180 0.030 0.050 0.020 0.010 0.070 0.040 0.130 0.320 0.180 0.050 0.020 P90 2.480 0.240 0.340 0.050 0.060 0.300 0.120 0.470 1.160 0.760 0.380 0.550 Max. 7.200 0.700 0.810 7.000 0.670 1.030 0.220 0.880 1.460 3.200 1. 700 3.190 P04 Min. 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 P50 0.007 0.019 0.011 0.000 0.000 0.000 0.000 0.000 0.000 0.010 0.010 0.020 P90 0.001 0.046 0.050 0.180 0.019 0.020 0.010 0.030 0.010 0.040 0.050 0.050 Max. 0.850 0.220 11.000 2.000 0.060 0.020 0.020 0.130 0.040 0.200 0.230 0.080 SAR·" P90 0.070 1.730 0.430 0.550 0.710 0.310 0.500 0.850 1.300 2.390 5.340 1.160 lrrig. Class 1 2 1 1 1 1 1 1 2 2 2 1 * Patched data (see Section G-A.3) ** Sodium adsorption ratio (see Section G-A.4) G-A-2 TDS vs Discharge for Magalakwena Riv- Plot o~ TDS vs. river discharge for (x 100) Magalakwena River 10 A A6Haas 8 .... " Cl E c o 6 ''; .;.J III L. .;.J C GI U C o u If) a I- 2 00 • Cl Cl ot:! 000 00 o o 30 60 90 120 150 River discharge (cumec) Plot of total dissolved salts tTDSJ vs. river discharge for Magalakwena R. 1000 B o A6Haas o .... . Cl Cl Cl 11 .. • 0 • ...... 00 100 a I- Cl ....o 10 0,01 0,1 10 100 100C log River discharge (cumec) l'i~Li.1:C v Time Series: TDS for ~fagalakwena Ri\ Time series plot or TDS concentration (x 100) ror the Ha~alakwena River at A6H009 A6Haa9 c 6 ....o ~ 111 I ~ C III 4 U oC u (f) Q f- 2 o 10/01171 02/26178 07 fZ4/84 12f20f90 Time series plot or TDS concentration CTDS] mg/l (x 100 ) and river discharge ror Magalakwena R. .-~. Flow cumec 10 150 B A6Haas 8 -4 U III 01 E "E J U c 6 o III .... Cl ~ I- 111 111 I .r. ~ U C III III 4 .... U '0 c: o I. u III ) (f) .... o er f- 2 +. ~,+ '. , :: '+ + +, ..+ .... + o ++++++++++++++++ I ' I I I I I I +++'1-1+++- - ++------+++-Ho+' 11/02177 05/27/78 12/19178 07/13179 Date (Month/Dau/Yaar) time the concentration increases for a given discharge range. If a trend is observed over a wide range of discharge and sufficient data is available, more detailed trend detection techniques would be used. The plot of the Mogalakwena data (the lower half of Figure G-A.1) was assessed with dates noted against each point, although these were omitted from the figure for reasons of clarity. This indicated that there was no discernable trend with time apparent in the data. To detect the change in chemical composition of river water the ratio of TDS concentration to chloride ion concentration is plotted as a time series and examined graphically using regression analysis. G·A.3 TECHNIQUE TO PATCH TDS DATA RECORDS FROM MEASURED CONDUCTIVITY VALUES. Many stations in South Africa have data sets with occasional missing values for the TDS. Using linear regression between TDS concentration and conductivity it is possible to patch the missing TDS data record, assuming sufficient pairs of TDS and conductivity are available. Table G-A.3 shows the linear regression coefficients used in the equation: TDS = Slope x EC + Intercept where: TDS is the TDS concentration EC is the electrical conductivity. Table G-A.2 MAR and TDS Load Estimates No River MAR MAR % of TDS load Load % of (m3106) rank total (tons) rank total 1 Marico 49.7 6 7 9205 6 7 2 Crocodile 204.8 1 29 47725 1 38 3 Notwane 24.3 7 3 10 902 5 9 4 Matlabas 20.8 8 3 390 10 1 5 Mokolo 116.9 2 17 5 515 7 4 6 Lephalala 98.7 4 14 13 838 4 11 7 Sub-Total 515.2 87575 8 Mogal:ik.'Wena 79.3 5 11 15 237 3 12 9 Motloutse 111.1 3 16 21 222 2 17 10 Total 705.6 - 100 124034 - 100 11 Limpopo at 412.0 - - 57026 -- Sterkloop Note: The mean annual runoff (MA.R) values are based on a sequence of 64 years of predicted runoff. The total dissolved salts (TDS) load estimates are based on mass balance calculations using the regression coefficients shown in Table G-AA with the monthly runoff. G-A-3 Table G·A.3 EC/TDS Linear Regression Coefficients River Sampling Intercept Slope R2 (%) station Crocodile A2H037 -23.9 7.5 97 Marico A3HOO7 -10.8 8.2 97 Matlabas A4HOO4 3.5 6.4 97 Tambotie A4HOO7 24.1 2.0 25 Hans Strijdom Dam A4ROOl 8.1 6.7 97 Hans Strijdom Weir A4ROOl 4.7 6.5 85 Limpopo· A5HOO6 3.2 6.4 93 Lephalala A5ROO2 3.4 6.5 99 Mogalakwena A6HOO9 14.8 6.3 99 Glen Alpine A6ROO2 19.9 6.0 99 Limpopo A7HOO4 15.4 6.0 75 Note: Linear regression coefficients calculated for the relationship between electrical conductivity (BC) and total dissolved salts (TDS) for in-fIlling data sets. Table G-A.4 TDS/Discharge Non-linear Regression Coefficients River Sampling A B R station Crocodile A2H037 486.0 -0.18* Marico A3HOO7 292.0 -0.14 0.5 Matlabas A4H004 30.0 -0.18 0.7 Tambotie A4HOO7 30.0 -0.04 0.4 Hans Strijdom A4ROO1 56.0 -0.05 0.4 Lephalala A5ROO2 154.0 -0.03* Limpopo A5HOO6 155.0 -0.02 003 Mogalakwena A6HOO9 357.0 -0.19 0.7 Limpopo A7HOO4 251.0 -0.08 0.8 Notes: * Coefficient estimated due to insufficient data. The non-linear regression equation is of the form: TDS =Act where Q is the discharge in m3/s. G-A-4 Table G-A.S Non-linear Regression Coefficients assumed for Rivers in Botswana River A B Notwane 536 -0.12 Bonwapitse 300 -0.12 Lotsane 300 -0.12 Motloutse 276 -0.12 G-A.4 SODIUM ADSORPTION RA no The sodium adsorption ratio (SAR) is used to determine whether the chemical composition of irrigation water will cause a breakdown of the colloidal structure of the soil caused by ion exchange reactions. This will cause a decrease in the permeability of the soil and result in poorer drainage. The SAR is calculated using the equation: SAR = -;;;~[N;:a=]/2::::;3;::;::;:;;: J[Ca]f20 + [Mg]f12 The measure of the SAR in combination with the electrical conductivity (EC) of the water can be used to classify the water in terms of fitness of use for irrigation. Figure G-A.3 shows a graphical presentation of the United States Department of Agriculture (USDA) classification (Richards, 1954). The figure is divided into 16 different sodium hazard classes ranging from Cl to C4, based on the SAR and EC value. The 16 classes proposed by the USDA have been simplified into three general classes of irrigation water. To determine the quality of water for irrigation purposes, both the SAR and EC values are used to position the water sample in one of the three irrigation classes. Table G-A.6 Classification of Water Quality for Irrigation Purposes Class USDA class Description 1 Cl-SI, C2-S1 Mostly suitable 2 C3-Sl, C4-Sl, Cl-S2, Increasing problems C2-S2, C3-S2 3 All other Mostly unsuitable G-A-5' G-A-6 Figure G-A.3 Classification of Irrigation Water 30 r Cl-S4 C2-S4 C3-S4 0 C4-S4 '''-; ..;....l a:l Cl-S3 ~ 20 ~ 0 '''-; ..;....l p... C2-S3 H 0 r:t.l '"d ~ C3-S3 S ...... ;::J '"d 10 0 C4-S3 lfl C4-S2 O~~~~~~~~~~~~LU~~~~~~~~~~~ 10 25 100 500 Electrical Conductivity (mS/m) LEGEND: t .. ::J Class 1 ~ Class 2 Class 3 from USDA (1954) APPENDIX G-B SAMPLE OUTPUTS OF RESERVOIR YIELD ANALYSIS LIST OF TABLES Page No G-B.l Output of Reservoir Simulation for CumberIand Dam Storing 750 m3IQ6 G-B-l G-B.2 Output of Reservoir Simulation for Buffelsdrift Dam Storing 750 m3 IQ6 G-B-3 G-B.3 Output of Reservoir Simulation for Sunnyside Dam Storing 1 000 m3 IQ6 G-B-5 G-B.4 Output of Reservoir Simulation for Selika Dam Storing 1 000 m3 IQ6 G-B-7 G-B.5 Output of Reservoir Simulation for Selika Dam Storing 1 000 m3 IQ6 with a Dam at Buffelsdrift G-B-9 G-B.6 Output of Reservoir Simulation for Selika Dam to Determine Reliability of Yields in the Early Years G-B-ll Table G-B.1 Output of Reservoir Simulation for Cumberland Dam Storing 750 m310' Cumber land reservoir simulations Parameters read from file : cumb.dat Cumber land inflows read from file F\CUMBFl with adjustment factor 1.000 Rainfalls read from file : R\MOIO-RAI with adjustment factor 1.000 Mont!> 1 2 3 4 5 6 7 8 9 10 11 12 Primary demand factor 1.00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 Secondary demand factor .89 1. 62 1. 95 1. 68 .83 .51 .17 .22 .59 1.42 1. 74 .37 Evaporation (mm) 177 . 223. 230. 220. 177. 167. 131. 109. 89. 80. 110. 145. Evaporation factor = 1. 000 Operating procedure applied in following months 0 N D J F M A M J J A S 1 1 1 1 1 1 1 1 1 1 1 1 When Cumberland resevoir less than 1.66 % full Reduced supply to be met = 80.0 % of demand Trigger for secondary demand when Cumber land 11.33 % full Crop planting in months 8 and 11 Cumber land Storage/Area/Stage curves Volume (mcm) .00 .10 10.10 80.30 263.60 661.10 1194.10 1448.00 2710.50 Area (km2) .00 .20 3.80 24.30 49.00 110.00 179.70 205.00 300.00 Elevation (m) 844.30 845.00 850.00 855.00 860.00 865.00 868.70 870.00 875.00 Volume (mcm) 4548.00 Area (km2) 435.00 Elevation (m) 880.00 Maximum reservoir storage 750.000 Minimum reservoir storage = 2.000 Initial conditions set by three year warm-up Starting from empty. with mean inflows. rainfalls etc Summary of results Demand (mem) Reliability (% of time) Monthly Yearly Target Actual Full 10.07 95.00 95.05 89.06 Reduced 8.06 99.00 99.09 95.31 Secondary 37.10 80.00 80.08 73.44 Deficit as proportion of primary demand 98.36% Deficit as proportion of secondary demand 79.53% Area transfer rule never applied G-B-l Table G-B.! (Continued) Annual results sununary for cumber land Demand Deficit Initial Inflow Spillage Transfer Evapo- Rainfall Releases Prime Second Prime Second volUme ration 1924 10.07 37.10 .00 23.67 439.31 518.00 115.03 .00 164.37 47.52 23.50 1925 10.07 37.10 .00 .00 701.94 .00 .00 .00 186.71 33.34 47.17 1926 10.07 37.10 .00 .00 501.40 .00 .00 .00 137.36 29.17 47.17 1927 10.07 37.10 .00 .00 346.04 14.20 .00 .00 98.86 18.66 47.17 1928 10.07 37.10 .00 .00 232.87 1.30 .00 .00 69.92 9.31 47.17 1929 10.07 37.10 .00 13 .43 126.38 2.40 .00 .00 46.08 10.96 33.74 1930 10.07 37.10 .00 37.10 59.92 .00 .00 .00 25.54 5.06 10.07 1931 10.07 37.10 .17 37.10 29.37 .00 .00 .00 12.55 3.32 9.90 1932 10.07 37.10 4.70 37.10 10.23 .00 .00 .00 3.75 .57 5.37 1933 10.07 37.10 1. 01 23.67 1.68 323.30 .00 .00 71.30 14.09 22.49 1934 10.07 37.10 .00 .00 245.27 .30 .00 .00 72.68 9.58 47.17 1935 10.07 37.10 .00 .00 135.30 184.60 .00 .00 62.54 10.83 47.17 1936 10.07 37.10 .00 .00 221.02 215.00 .00 .00 96.47 25.68 47.17 1937 10.07 37.10 .00 .00 318.05 162.90 .00 .00 117 .25 25.81 47.17 1938 10.07 37.10 .00 .00 342.34 681.30 171.88 .00 159.92 39.72 47.17 1939 10.07 37.10 .00 .00 684.39 48.50 .00 .00 195.74 60.65 47.17 1940 10.07 37.10 .00 .00 550.64 132.80 .00 .00 174.45 49.23 47.17 1941 10.07 37.10 .00 .00 511.05 17 .50 .00 .00 139.74 19.66 47.17 1942 10.07 37.10 .00 .00 361.30 182.90 .00 .00 122.54 30.96 47.17 1943 10.07 37.10 .00 .00 405.46 1805.40 1333.38 .00 181.14 62.85 47.17 1944 10.07 37.10 .00 .00 712.02 359.30 167.08 .00 218.90 46.40 47.17 1945 10.07 37.10 .00 .00 684.57 524.30 329.72 .00 210.57 48.95 47.17 1946 10.07 37.10 .00 .00 670.35 .00 .00 .00 177 .16 21.19 47.17 1947 10.07 37.10 .00 .00 467.21 93.90 .00 .00 139.78 42.29 47.17 1948 10.07 37.10 .00 .00 416.45 50.20 .00 .00 122.30 20.34 47.17 1949 10.07 37.10 .00 .00 317 .52 44.10 .00 .00 97.93 18.31 47.17 1950 10.07 37.10 .00 .00 234.83 .30 .00 .00 70.83 16.26 47.17 1951 10.07 37.10 .00 13 .43 133.39 .00 .00 .00 47.59 10.30 33.74 1952 10.07 37.10 .00 30.20 62.35 47.40 .00 .00 36.90 9.26 16.97 1953 10.07 37.10 .00 37.10 65.15 .00 .00 .00 28.39 6.21 10.07 1954 10.07 37.10 .00 23.67 32.90 1157.70 412.87 .00 118.95 39.41 23.50 1955 10.07 37.10 .00 .00 674.68 279.30 66.40 .00 210.08 47.97 47.17 1956 10.07 37.10 .00 .00 678.30 51. 60 .00 .00 186.06 51.39 47.17 1957 10.07 37.10 .00 .00 548.06 40.20 .00 .00 155.05 34.00 47.17 1958 10.07 37.10 .00 .00 420.04 175.80 .00 .00 150.75 51. 96 47.17 1959 10.07 37.10 .00 .00 449.88 .00 .00 .00 123.10 23.20 47.17 1960 10.07 37.10 .00 .00 302.81 244.50 .00 .00 123.23 42.46 47.17 1961 10.07 37.10 .00 .00 419.36 .00 .00 .00 115.33 20.42 47.17 1962 10.07 37.10 .00 .00 277 .28 .00 .00 .00 80.07 13 .56 47.17 1963 10.07 37.10 .00 .00 163.61 .00 .00 .00 54.52 9.33 47.17 1964 10.07 37.10 .00 13 .43 71.25 .00 .00 .00 23.04 4.30 33.74 1965 10.07 37.10 1.34 37.10 18.76 .00 .00 .00 7.34 .84 8.73 1966 10.07 37.10 1.52 23.67 3.54 2082.40 1289.45 .00 123.18 45.97 21. 98 1967 10.07 37.10 .00 .00 697.30 .00 .00 .00 184.64 29.86 47.17 1968 10.07 37.10 .00 .00 495.35 .00 .00 .00 134.17 23.98 47.17 ,': 1969 10.07 37.10 .00 .00 337.99 1. 00 .00 .00 94.81 14.76 47.17 1970 10.07 37.10 .00 .00 211.77 451.70 .00 .00 129.80 31. 51 47.17 1971 10.07 37.10 .00 .00 518.02 306.20 .00 .00 190.34 48.97 47.17 1972 10.07 37.10 .00 .00 635.68 .00 .00 .00 168.56 20.15 47.17 1973 10.07 37.10 .00 .00 440.10 100.50 .00 .00 137.88 40.32 47.17 1974 10.07 37.10 .00 .00 395.87 852.60 394.53 .00 175.36 56.36 47.17 1975 10.07 37.10 .00 .00 687.77 1218.00 996.06 .00 215.23 66.52 47.17 1976 10.07 37.10 .00 .00 713.82 459.20 289.73 .00 221. 71 70.16 47.17 1977 10.07 37.10 .00 .00 684.56 1492.10 1259.85 .00 214.45 53.40 47.17 1978 10.07 37.10 .00 .00 708.59 .00 .00 .00 189.09 32.72 47.17 1979 10.07 37.10 .00 .00 505.06 147.60 .00 .00 165.52 49.06 47.17 1980 10.07 37.10 .00 .00 489.03 201.90 .00 .00 165.15 44.72 47.17 1981 10.07 37.10 .00 .00 523.33 3.40 .00 .00 141.72 20.17 47.17 1982 10.07 37.10 .00 .00 358.00 .00 .00 .00 99.74 14.41 47.17 1983 10.07 37.10 .00 .00 225.51 .00 .00 .00 68.26 10.25 47.17 1984 10.07 37.10 .00 13 .43 120.32 .00 .00 .00 42.66 6.86 33.74 1985 10.07 37.10 .00 37.10 50.78 .00 .00 .00 21. 79 4.49 10.07 1986 10.07 37.10 .84 37.10 23.41 .50 .00 .00 9.90 2.20 9.23 1987 10.07 37.10 1. 01 37.10 6.98 52.50 .00 .00 12.81 2.14 9.06 Mean 10.07 37.10 .17 7.43 230.13 106.66 .00 117.87 27.72 39.58 G-B-2 Table G-B.2 Output of Reservoir Simulation for Buffelsdrift Dam Storing 750 mJIO' Buffelsdrift reservoir simulations Parameters read from file: buff.dat Buffelsdrift inflows read from file F\BUFFF1 with adjustment factor 1.000 Rainfalls read from file : R\M020-RAI with adjustment factor 1.000 Month 1 2 3 4 5 6 7 8 9 10 11 12 Primary demand factor 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1.00 1. 00 1. 00 1. 00 1. 00 Secor:dary demand factor .89 1. 62 1. 95 1. 68 .83 .51 .17 .22 .59 1.42 1. 74 .37 Evaporation (mm) 177. 223. 230. 220. 177 . 167. 131. 109. 89. 80. 110. 145. E'Japoration factor = 1. 000 Operating procedure applied in following months o N D J F M A M J J A S 1 1 1 111 1 1 1 1 1 1 When Buffelsdrift resevoir less than 1.89 % full Reduced supply to be met = 80.0 % of demand Trigger for secondary demand when Buffelsdrift 15.14 % full Crop planting in months 8 and 11 Buffelsdrift Storage/Area/Stage curves Volume (mcm) .00 1. 70 21. 70 132.80 399.80 895.70 1663.00 2847.00 4065.60 Area (km2) .00 .80 8.50 34.60 73.60 123.50 184.60 289.00 373.30 Elevation (m) 831.00 835.00 840.00 845.00 850.00 855.00 860.00 865.00 868.70 Volume (mcm) Area (km2) Elevation (m) Maximum reservoir storage 750.000 Minimum reservoir storage 2.000 Initial conditions set by three year warm-up Starting f~om empty, with mean inflOWS, rainfalls etc SUll'J::',ary of results Demand (mcm) Reliability (% of time) Monthly Yearly Target Actual Full 13 .60 95.00 95.05 87.50 Reduced 10.88 99.00 99.09 96.88 Secondary 52.08 80.00 80.34 71.88 Deficit as proportion of primary demand 98.28% Deficit as proportion of secondary demand 79.80% Area transfer rule never applied G-B-3 Table G-B.2 (Continued) Annual results summary for Buffelsdrifc Demand Deficit Initial Inflow Spillage Transfer Evapo- Rainfall Releases Prime Second Prime Second volume ration 1924 13.60 52.08 .00 33.23 471.49 557.20 181.95 .00 165.15 51.38 32.45 1925 13 .60 52.08 .00 .00 700.52 9.90 .00 .00 174.34 28.05 65.68 1926 13.60 52.08 .00 .00 498.45 5.30 .00 .00 138.57 27.76 65.68 1927 13 .60 52.08 .00 .00 327.26 37.00 .00 .00 103.17 21. 65 65.68 1928 13 .60 52.08 .00 .00 217 . 06 12.80 .00 .00 71.79 13 .60 65.68 1929 13.60 52.08 .00 18.85 105.99 16.10 .00 .00 37.73 10.26 46.83 1930 13.60 52.08 .00 52.08 47.79 3.90 .00 .00 21.73 5.15 13.60 1931 13.60 52.08 .91 52.08 21.51 8.20 .00 .00 11.30 2.70 12.69 1932 13.60 52.08 8.16 52.08 8.43 1.20 .00 .00 3.09 .47 5.44 1933 13 .60 52.08 1.36 33.23 1.56 373.20 .00 .00 84.30 17.12 31.09 1934 13 .60 52.08 .00 .00 276.49 17.70 .00 .00 87.29 13 .64 65.68 1935 13.60 52.08 .00 .00 154.86 204.50 .00 .00 71.58 12.00 65.68 1936 13.60 52.08 .00 .00 234.10 260.80 .00 .00 109.33 24.89 65.68 1937 13 .60 52.08 .00 .00 344.78 171.20 .00 .00 129.00 27.96 65.68 1938 13.60 52.08 .00 .00 349.27 735.30 219.15 .00 154.13 37.82 65.68 1939 13.60 52.08 .00 .00 683.43 58.50 .00 .00 180.10 52.85 65.68 1940 13 .60 52.08 .00 .00 549.00 144.20 .00 .00 165.36 42.32 65.68 1941 13 .60 52.08 .00 .00 504.48 30.30 .00 .00 141. SO 27.16 65.68 1942 13.60 52.08 .00 .00 354.46 208.20 .00 .00 127.30 29.93 65.68 1943 13 .60 52.08 .00 .00 399.61 1887.30 l400.42 .00 171.12 62.48 65.68 1944 13 .60 52.08 .00 .00 712.18 443.50 248.81 .00 196.68 38.66 65.68 1945 13 .60 52.08 .00 .00 683.18 536.60 335.58 .00 190.60 41.72 65.68 1946 13 .60 52.08 .00 .00 669.64 3.20 .00 .00 167.79 23.11 65.68 1947 13.60 52.08 .00 .00 462.49 95.40 .00 .00 140.37 43.04 65.68 1948 13.60 52.08 .00 .00 394.88 54.90 .00 .00 121.91 19.60 65.68 1949 13 .60 52.08 .00 .00 281.79 56.00 .00 .00 96.54 19.56 65.68 1950 13.60 52.08 .00 .00 195.12 90.20 .00 .00 79.50 17.10 65.68 1951 13.60 52.08 .00 18.85 157.24 2.30 .00 .00 52.63 9.99 46.83 1952 13 .60 52.08 .00 33.23 70.08 107.90 .00 .00 51.74 15.47 32.45 1953 13.60 52.08 .00 18.85 109.26 10.60 .00 .00 37.01 8.42 46.83 1954 13.60 52.08 .00 33.23 44.45 1223.40 485.12 .00 111.16 35.78 32.45 1955 13.60 52.08 .00 .00 674.90 307.00 91.95 .00 190.49 43.87 65.68 1956 13 .60 52.08 .00 .00 677.65 66.10 .00 .00 173.96 47.55 65.68 1957 13 .60 52.08 .00 .00 551. 66 52.90 .00 .00 153.91 34.87 65.68 1958 13.60 52.08 .00 .00 419.84 192.70 .00 .00 149.79 44.01 65.68 1959 13.60 52.08 .00 .00 441.09 5.80 .00 .00 126.01 23.29 65.68 1960 13 .60 52.08 .00 .00 278.48 274.30 .00 .00 124.70 42.31 65.68 1961 13 .60 52.08 .00 .00 404.72 1.40 .00 .00 116.40 20.10 65.68 1962 13 .60 52.08 .00 .00 244.14 2.50 .00 .00 77.41 15.28 65.68 1963 13 .60 52.08 .00 18.85 118.83 1. 80 .00 .00 38.71 6.92 46.83 1964 13 .60 52.08 .00 52.08 42.02 3.20 .00 .00 19.25 3.54 13 .60 1965 13 .60 52.08 1.13 52.08 15.90 19.00 .00 .00 11.20 .98 12.47 1966 13 .60 52.08 .91 33.23 12.22 2181. 50 1392.24 .00 115.51 41. 60 31. 54 1967 13.60 52.08 .00 .00 696.02 22.40 .00 .00 173.81 31.13 65.68 1968 13 .60 52.08 .00 .00 510.06 2.00 .00 .00 139.S9 25.68 65.68 1969 13 .60 52.08 .00 .00 332.17 6.00 .00 .00 99.56 16.58 65.68 1970 13 .60 52.08 .00 .00 189.51 462.80 .00 .00 123.57 30.55 65.68 1971 13 .60 52.08 .00 .00 493.61 362.10 11.14 .00 175.89 37.15 65.68 1972 13.60 52.08 .00 .00 640.15 3.40 .00 .00 162.24 17 .29 65.68 1973 13 .60 52.08 .00 .00 432.93 121.20 .00 .00 140.77 38.90 65.68 1974 13 .60 52.08 .00 .00 386.58 888.80 414.32 .00 165.77 56.66 65.68 1975 13 .60 52.0S .00 .00 686.26 1242.30 1015.98 .00 194.02 63.73 65.68 1976 13.60 52.08 .00 .00 716.61 498.20 322.72 .00 198.95 57.06 65.68 1977 13 .60 52.08 .00 .00 684.52 1605.10 1375.63 .00 193.64 52.68 65.68 1978 13.60 52.08 .00 .00 707.35 .60 .00 .00 174.60 28.95 65.68 1979 13 .60 52.08 .00 .00 496.62 154.50 .00 .00 157.52 42.74 65.68 1980 13.60 52.08 .00 .00 470.66 241.40 .00 .00 159.68 49.24 65.68 1981 13 .60 52.08 .00 .00 535.94 5.20 .00 .00 146.09 31.11 65.68 1982 13 .60 52.08 .00 .00 360.48 5.60 .00 .00 106.97 18.37 65.68 1983 13.60 52.08 .00 .00 211. 79 6.80 .00 .00 69.58 13 .84 65.68 1984 13 .60 52.08 .00 18.85 97.17 10.60 .00 .00 32.91 5.18 46.83 1985 13 .60 52.08 .23 52.08 33.21 6.10 .00 .00 16.10 2.64 13 .37 1986 13 .60 52.08 1.59 52.08 12.48 14.90 .00 .00 10.83 2.34 12.01 1987 13 .60 52.08 .68 33.23 6.88 146.40 .00 .00 31. 64 6.84 31.77 Mean 13.60 52.08 .23 10.2S 254.40 117.11 .00 115.09 27.10 55.16 G-B-4 Table G-B.3 Output of Reservoir Simulation for Sunnyside Dam Storing 1 000 m3 10' Sunnyside reservoir simulations Parameters read from file : sun.dat sunnyside inflows read from file F\Su~F1 with adjustment factor 1.000 Rainfalls read from file : R\M030-RAI with adjustment factor 1.000 Month 1 2 3 4 5 6 7 8 9 10 11 12 Primary demand factor 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 Secondary demand factor .89 1. 62 1. 95 1. 68 .83 .51 .17 .22 .59 1. 42 1. 74 .37 Evaporation (mm) 182. 21:;. 212. 211. 176. 166. 130. Ill. 91. 82. 111. 150. Evaporation factor = 1. 000 Operating procedure applied in following months 0 N D J F M A M J J A S 1 1 1 1 1 1 1 1 1 1 1 1 when Sunnyside resevoir less than 2.83 % full Reduced supply to be met = 80.0 % of demand Trigger for secondary demand when sunnyside 13.33 % full Crop planting in months 8 and 11 Sunnyside Storage/Area/Stage curves Volume (mcm) .00 1.20 16.20 78.70 269.70 674.40 1428.20 2868.20 Area (km2) .00 1.00 5.00 20.00 56.40 105.50 196.00 380.00 Elevation (m) 772.70 775.00 780.00 785.00 790.00 795.00 800.00 805.00 Maximum reservoir storage 1000.000 Minimum reservoir storage = 2.000 Initial conditions set by three year warm-up Starting from empty, with mean inflows, rainfalls etc SWTh~ary of results Demand (mc:n) Reliability (% of time) Monthly Yearly Target Actual Full 24.30 95.00 95.05 90.63 Reduced 19.44 99.00 99.09 93.75 Secondary 65.40 80.00 81.12 71.88 Deficit as proportion of primary demand 98.34% Deficit as proportion of secondary demand 80.22% Area transfer rule never applied G-B-5 Table G-B.3 (Continued) Annual results sununary for sunnyside Demand Deficit Initial Inflow Spillage Transfer Evapo- Rainfall ?~eleases Prime Second Prime Second volume ration 1924 24.30 65.40 .00 41. 73 639.70 947.80 461.97 .00 211.38 62.90 47.97 1925 24.30 65.40 .00 .00 929.08 21. 00 .00 .00 219.74 29.97 89.70 1926 24.30 65.40 .00 .00 670.61 34.70 .00 .00 170.64 30.28 89.70 1927 24.30 65.40 .00 .00 475.24 21. 30 .00 .00 130.61 28.94 89.70 1928 24.30 65.40 .00 .00 305.17 1. 50 .00 .00 87.69 20.47 89.70 1929 24.30 65.40 .00 23.67 149.76 5.30 .00 .00 42.53 12.47 66.03 1930 24.30 65.40 .00 65.40 58.96 55.30 .00 .00 29.40 7.05 24.30 1931 24.30 65.40 .00 65.40 67.61 .00 .00 .00 22.78 4.66 24.30 1932 24.30 65.40 7.31 65.40 25.20 .00 .00 .00 7.51 1.13 16.99 1933 24.30 65.40 2.43 41.73 1. 83 271.90 .00 .00 62.47 12.16 45.54 1934 24.30 65.40 .00 23.67 177.87 .00 .00 .00 49.60 8.79 66.03 1935 24.30 65.40 .00 41. 73 71. 03 179.40 .00 .00 44.32 6.15 47.97 1936 24.30 65.40 .00 .00 164.28 496.90 .00 .00 113.24 18.73 89.70 1937 24.30 65.40 .00 .00 476.98 179.60 .00 .00 153.07 31.16 89.70 1938 24.30 65.40 .00 .00 444.97 839.80 145.59 .00 191.78 41.32 89.70 1939 24.30 65.40 .00 .00 899.01 171.70 .00 .00 239.69 62.58 89.70 1940 24.30 65.40 .00 .00 803.90 472.70 146.29 .00 243.62 52.04 89.70 1941 24.30 65.40 .00 .00 849.03 65.40 .00 .00 210.27 48.08 89.70 1942 24.30 65.40 .00 .00 662.54 165.70 .00 .00 179.43 37.12 89.70 1943 24.30 65.40 .00 .00 596.24 2179.50 1602.39 .00 217.79 75.61 89.70 1944 24.30 65.40 .00 .00 941.46 338.60 82.94 .00 249.24 42.47 89.70 1945 24.30 65.40 .00 .00 900.65 1066.30 786.49 .00 248.45 48.70 89.70 1946 24.30 65.40 .00 .00 891.01 .00 .00 .00 211.42 31. 30 89.70 1947 24.30 65.40 .00 .00 621.19 140.90 .00 .00 171. 60 50.00 89.70 1948 24.30 65.40 .00 .00 550.80 55.00 .00 .00 150.01 21. 94 89.70 1949 24.30 65.40 .00 .00 388.02 12.00 .00 .00 111. 44 23.29 89.70 1950 24.30 65.40 .00 11. 51 222.17 31. 50 .00 .00 67.77 12.62 78.19 1951 24.30 65.40 .00 65.40 120.33 .90 .00 .00 41.47 6.91 24.30 1952 24.30 65.40 .00 41.73 62.36 437.80 .00 .00 79.18 21. 69 47.97 1953 24.30 65.40 .00 .00 394.70 80.60 .00 .00 121. 41 29.07 89.70 1954 24.30 65.40 .00 .00 293.26 1901.30 1085.61 .00 180.58 60.50 89.70 1955 24.30 65.40 .00 .00 899.17 548.30 269.19 .00 248.81 53.74 89.70 1956 24.30 65.40 .00 .00 893.51 44.30 .00 .00 215.47 53.28 89.70 1957 24.30 65.40 .00 .00 685.92 59.60 .00 .00 179.52 51. 01 89.70 1958 24.30 65.40 .00 .00 527.30 207.20 .00 .00 167.66 40.98 89.70 1959 24.30 65.40 .00 .00 518.13 .00 .00 .00 135.96 19.70 89.70 1960 24.30 65.40 .00 .00 312.17 572.70 .00 .00 166.22 49.42 89.70 1961 24.30 65.40 .00 .00 678 .37 .00 .00 .00 169.25 31.20 89.70 1962 24.30 65.40 .00 .00 450.62 .00 .00 .00 . 123.95 31.97 89.70 1963 24.30 65.40 .00 .00 268.94 .00 .00 .00 76.48 13 .33 89.70 1964 24.30 65.40 .00 23.67 116.09 .00 .00 .00 29.27 6.29 66.03 1965 24.30 65.40 6.35 65.40 27.08 .00 .00 .00 8.18 1. 04 17.95 1966 24.30 65.40 6.48 41.73 2.00 2177.00 1098.88 .00 145.98 31.24 41. 49 1967 24.30 65.40 .00 .00 923.88 .00 .00 .00 218.80 40.89 89.70 1968 24.30 65.40 .00 .00 656.27 .00 .00 .00 165.50 37.63 89.70 1969 24.30 65.40 .00 .00 438.70 .00 .00 .00 119.79 19.68 89.70 1970 24.30 65.40 .00 .00 248.90 649.30 .00 .00 153.43 33.90 89.70 1971 24.30 65.40 .00 .00 688.97 534.00 31.47 .00 229.54 45.65 89.70 1972 24.30 65.40 .00 .00 917.91 .00 .00 .00 216.81 31.16 89.70 1973 24.30 65.40 .00 .00 642.57 287.60 .00 .00 193.04 50.03 89.70 1974 24.30 65.40 .00 .00 697.46 1342.70 831.78 .00 231.29 59.28 89.70 1975 24.30 65.40 .00 .00 946.67 1627.00 1348.24 .00 256.29 78.81 89.70 1976 24.30 65.40 .00 .00 958.26 792.10 537.06 .00 257.44 75.93 89.70 1977 24.30 65.40 .00 .00 942.10 1826.00 1556.13 .00 254.75 75.72 89.70 1978 24.30 65.40 .00 .00 943.24 .30 .00 .00 223.08 43.76 89.70 1979 24.30 65.40 .00 .00 674.52 349.30 .00 .00 205.70 40.54 89.70 1980 24.30 65.40 .00 .00 768.97 446.00 46.71 .00 234.15 58.58 89.70 1981 24.30 65.40 .00 .00 903.00 2.90 .00 .00 216.35 46.88 89.70 1982 24.30 65.40 .00 .00 646.73 .00 .00 .00 164.62 38.40 89.70 1983 24.30 65.40 .00 .00 430.80 2.00 .00 .00 119.20 25.21 89.70 1984 24.30 65.40 .00 11. 51 249.11 .00 .00 .00 71.05 12.70 78.19 .. 00 1985 24.30 65.40 65.40 112.57 .00 .00 .00 38.16 5.02 24.30 1986 24.30 65.40 1. 62 65.40 55.14 .00 .00 .00 18.36 3.95 22.68 1987 24.30 65.40 1. 62 41.73 18.06 177.50 .00 .00 34.82 3.36 46.35 Mean 24.30 65.40 .40 12.53 340.94 156.73 .00 149.20 33.60 76.76 G-B-6 Table G-B.4 Output of Reservoir Simulation for Selika Dam Storing 1 000 m310 6 Selika reservoir simulations Parameters read from file: sel.dat selika inflows read from file F\SELFl with adjustment factor 1.000 Rainfalls read from file : R\M040-RAI with adjustment factor 1.000 Month 1 2 3 4 5 6 7 8 9 10 11 12 primary demand factor 1. 00 1. 00 1.00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1.00 Secondary demand factor .89 1. 62 1. 95 1.68 .83 .51 .17 .22 .59 1. 42 1. 74 .37 Evaporation (mm) 179. 208. 208. 207. 172 . 163. 128. 109. 90. 81. 108. 147. Evaporation factor = 1. 000 Operating procedure applied in following months 0 N D J F M A M J J A S 1 1 1 1 1 1 1 1 1 1 1 1 When Selika resevoir less than 2.30 % full Reduced supply to be met = 80.0 % of demand Trigger for secondary demand when Selika 12.52 % full Crop planting in months 8 and 11 Selika storage/Area/Stage curves Volume (mcm) .00 .04 5.00 42.70 238.80 773.50 1790.00 3427.00 Area (km2) .00 .05 1. 90 16.30 62.30 150.60 255.00 402.20 Elevation (m) 767.00 770.00 775.00 780.00 785.00 790.00 795.00 800.00 Maximum reservoir storage 1000.000 Minimum reservoir storage = 2.000 Initial conditions set by three year warm-up Starting from empty, with mean inflows, rainfalls etc Summary of results Demand (mcm) Reliability (% of time) Monthly Yearly Target Actual Full 24.50 95.00 95.05 89.06 Reduced 19.60 99.00 99.09 93.75 Secondary 74.46 80.00 81.12 70.31 Deficit as proportion of primary demand 98.40% Deficit as proportion of secondary demand 80.31% Area transfer rule never applied G-B-7 Table G-B.4 (Continued) Annual results summary for Selika Demand Deficit Initial Inflow Spillage Transfer Evapo- Rainfall Releases Prime Second Prime Second volume ration 1924 24.50 74.46 .00 47.51 734.85 1199.00 757.46 .00 271.63 65.52 51.45 1925 24.50 74.46 .00 .00 918.83 10.70 .00 .00 265.43 58.40 98.96 1926 24.50 74.46 .00 .00 623.54 21. 40 .00 .00 189.32 23.30 98.96 1927 24.50 74.46 .00 .00 379.96 116.20 .00 .00 141.97 25.07 98.96 1928 24.50 74.46 .00 .00 280.30 20.60 .00 .00 95.88 23.16 98.96 1929 24.50 74.46 .00 26.95 129.22 29.80 .00 .00 48.13 18.45 72 .01 1930 24.50 74.46 .00 47.51 57.33 141.70 .00 .00 52.94 12.14 51.45 1931 24.50 74.46 1.22 26.95 106.77 .00 .00 .00 29.80 8.01 70.78 1932 24.50 74.46 10.81 74.46 14.19 4.80 .00 .00 4.94 1.38 13 .69 1933 24.50 74.46 2.45 47.51 1. 75 256.00 .00 .00 68.13 7.70 49.00 1934 24.50 74.46 .00 26.95 148.32 .00 .00 .00 46.64 9.30 72 .01 1935 24.50 74.46 .00 47.51 38.97 193.50 .00 .00 43.60 4.51 51. 45 1936 24.50 74.46 .00 .00 141. 92 721.70 .00 .00 158.73 28.91 98.96 1937 24.50 74.46 .00 .00 634.83 215.70 .00 .00 229.93 41. 36 98.96 1938 24.50 74.46 .00 .00 563.01 914.20 312.58 .00 250.53 66.16 98.96 1939 24.50 74.46 .00 .00 881.29 278 .10 .00 .00 296.77 66.89 98.96 1940 24.50 74.46 .00 .00 830.56 714.00 354.87 .00 297.56 71.27 98.96 1941 24.50 74.46 .00 .00 864.43 66.40 .00 .00 256.68 41.15 98.96 1942 24.50 74.46 .00 .00 616.34 185.10 .00 .00 205.72 39.11 98.96 1943 24.50 74.46 .00 .00 535.87 2260.30 1551.55· .00 252.02 33.06 98.96 1944 24.50 74.46 .00 .00 926.69 307.60 .00 .00 291.22 24.51 98.96 1945 24.50 74.46 .00 .00 868.63 1254.20 924.93 .00 295.22 69.62 98.96 1946 24.50 74.46 .00 .00 873.33 .00 .00 .00 252.69 42.55 98.96 1947 24.50 74.46 .00 .00 564.23 197.00 .00 .00 199.51 60.92 98.96 1948 24.50 74.46 .00 .00 523.68 146.70 .00 .00 187.63 38.61 98.96 1949 24.50 74.46 .00 .00 422.40 .80 .00 .00 135.11 23.21 98.96 1950 24.50 74.46 .00 26.95 212.34 23.80 .00 .00 73.12 16.49 72 .01 1951 24.50 74.46 .00 74.46 107.49 4.50 .00 .00 45.80 7.42 24.50 1952 24.50 74.46 .00 47.51 49.12 623.50 .00 .00 113.84 23.05 51.45 1953 24.50 74.46 .00 .00 530.38 127.50 .00 .00 182.84 34.49 98.96 1954 24.50 74.46 .00 .00 410.58 2317.70 1587.77 .00 231. 00 77.77 98.96 1955 24.50 74.46 .00 .00 888.32 567.10 254.02 .00 295.57 66.65 98.96 1956 24.50 74.46 .00 .00 873.52 28.70 .00 .00 258.53 68.06 98.96 1957 24.50 74.46 .00 .00 612.78 72 .80 .00 .00 198.81 42.37 98.96 1958 24.50 74.46 .00 .00 430.18 292.90 .00 .00 192.71 50.56 98.96 1959 24.50 74.46 .00 .00 481. 98 9.50 .00 .00 151.92 24.22 98.96 1960 24.50 74.46 .00 .00 264.81 723.20 .00 .00 210.07 51. 40 98.96 1961 24.50 74.46 .00 .00 730.38 .00 .00 .00 215.56 32.10 98.96 1962 24.50 74.46 .00 .00 447.95 20.00 .00 .00 147.24 33.45 98.96 1963 24.50 74.46 .00 13 .10 255.20 .00 .00 .00 84.09 14.79 85.86 1964 24.50 74.46 .00 74.46 100.05 .70 .00 .00 41. 53 5.83 24.50 1965 24.50 74.46 2.86 74.46 40.54 .00 .00 .00 15.52 2.53 21.64 1966 24.50 74.46 3.19 47.51 5.92 2435.80 1345.05 .00 177.31 38.03 48.26 1967 24.50 74.46 .00 .00 909.13 .00 .00 .00 258.50 32.91 98.96 1968 24.50 74.46 .00 .00 584.59 145.10 .00 .00 197.70 59.31 98.96 1969 24.50 74.46 .00 .00 492.33 136.50 .00 .00 181. 66 38.74 98.96 1970 24.50 74.46 .00 .00 386.96 683.60 .00 .00 225.51 50.72 98.96 1971 24.50 74.46 .00 .00 796.80 786.20 382.08 .00 291.38 89.66 98.96 1972 24.50 74.46 .00 .00 900.25 .00 .00 .00 257.71 36.38 98.96 1973 24.50 74.46 .00 .00 579.96 385.40 .00 .00 233.73 48.66 98.96 1974 24.50 74.46 .00 .00 681.33 1645.10 1096.69 .00 278.73 75.89 98.96 1975 24.50 74.46 .00 .00 927.95 1865.90 1509.53 .00 304.67 57.49 98.96 1976 24.50 74.46 .00 .00 938.17 737.80 427.29 .00 299.97 67.80 98.96 1977 24.50 74.46 .00 .00 917 .56 1839.60 1484.54 .00 300.80 46.84 98.96 1978 24.50 74.46 .00 .00 919.70 .00 .00 .00 262.55 36.32 98.96 1979 24.50 74.46 .00 .00 594.52 593.90 27.34 .00 254.68 71. 84 98.96 1980 24.50 74.46 .00 .00 879.28 678.20 368.74 .00 301.12 86.13 98.96 1981 24.50 74.46 .00 .00 874.80 .00 .00 .00 253.13 37.87 98.96 1982 24.50 74.46 .00 .00 560.58 .00 .00 .00 172.24 29.26 98.96 1983 24.50 74.46 .00 .00 318.64 11.50 .00 .00 106.68 16.88 98.96 1984 24.50 74.46 .00 13 .10 141.38 89.70 .00 .00 59.81 9.57 85.86 1985 24.50 74.46 .00 74.46 94.98 .00 .00 .00 39.57 6.99 24.50 1986 24.50 74.46 2.86 74.46 37.91 .00 .00 .00 14.60 3.50 21.64 1987 24.50 74.46 1.76 47.51 5.16 275.60 .00 .00 61. 55 17 .48 49.69 Mean 24.50 74.46 .39 14.27 412.15 193.51 .00 180.62 37.71 84.30 G-B-8 Table G-B.S Output of Reservoir Simulation for Selika Dam Storing 1 000 mJ I0' with a Dam at Buffelsdrift selika reservoir simulations Parameters read from file: sel.dat Selika inflows read from file F\SELF5 with adjustment factor 1.000 Rainfalls read from file : R\M040-RAI with adjustment factor 1.000 Month 1 2 3 4 5 5 7 8 9 10 11 12 Primary demand factor 1. 00 1. 00 1.00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 Secondary demand factor .89 1. 62 1.95 1. 68 .83 .51 .17 .22 .59 1.42 1. 74 .37 Evaporation (mm) 179. 208. 208. 207. 172 . 163. 128. 109. 90. 81. 108. 147. Evaporation factor = 1. 000 Operating procedure applied in following months o N D J F M A M J J A S 111 1 1 1 1 1 1 1 1 1 When Selika resevoir less than 1.56 % full Reduced supply to be met = 80.0 % of demand Trigger for secondary demand when Selika 13.28 % full Crop planting in months 8 and 11 selika Storage/Area/Stage curves Volume (mcm) .00 .04 5.00 42.70 238.80 773.50 1790.00 3427.00 Area (km2) .00 .05 1.90 16.30 62.30 150.60 255.00 402.20 Elevation (m) 767.00 770.00 775.00 780.00 785.00 790.00 795.00 800.00 Maximum reservoir storage 1000.000 Minimum reservoir storage = 2.000 Initial conditions set by three year warm-up Starting from empty, with mean inflows, rainfalls erc Summary of results Demand (mcm) Reliability (% of time) Monthly Yearly Target Actual Full 13 .37 95.00 95.31 90.63 Reduced 10.70 99.00 99.09 96.88 Secondary 59.57 80.00 80.47 71. 88 Deficit as proportion of primary demand 98.39% Deficit as proportion of secondary demand 79.91% Area transfer rule never applied G-B-9 Table G-B.S (Continued) Annual results summary for Selika Demand Deficit Initial Inflow Spillage Transfer Evapo- Rainfall Releases Prime Second Prime Second volume ration 1924 13 .37 59.57 .00 38.01 570.18 831. 3 0 255.87 .00 244.47 56.75 34.93 1925 13 .37 59.57 .00 .00 922.95 10.70 .00 .00 269.47 59.20 72 .94 1926 13.37 59.57 .00 .00 650.45 21.40 .00 .00 199.73 24.49 72.94 1927 13 .37 59.57 .00 .00 423.67 116.20 .00 .00 156.93 27.61' 72.94 1928 13 .37 59.57 .00 .00 337.61 20.60 .00 .00 118.85 28.50 72.94 1929 13.37 59.57 .00 10.48 194.92 29.80 .00 .00 76.59 28.84 62.46 1930 13 .37 59.57 .00 38.01 114.51 141.70 .00 .00 75.70 17 .10 34.93 1931 13 .37 59.57 .00 21.56 162.68 .00 .00 .00 56.70 14.50 51. 38 1932 13.37 59.57 .00 59.57 69.10 4.80 .00 .00 33.76 8.08 13.37 1933 13 .37 59.57 .22 59.57 34.85 4.10 .00 .00 16.74 3.62 13 .15 1934 13 .37 59.57 5.79 59.57 12.69 .00 .00 .00 4.34 .98 7.58 1935 13 .37 59.57 4.90 59.57 1.76 21.40 .00 .00 5.80 .SO 8.47 1936 13.37 59.57 .45 38.01 9.70 533.80 .00 .00 101. 02 16.39 34.49 1937 13 .37 59.57 .00 .00 424.38 107.80 .00 .00 158.84 28.42 72.94 1938 13 .37 59.57 .00 .00 328.82 581.60 .00 .00 190.48 44.62 72.94 1939 13 .37 59.57 .00 .00 691. 62 247.50 .00 .00 256.75 57.81 72.94 1940 13 .37 59.57 .00 .00 667.24 598.10 103.98 .00 284.76 67.S3 72.94 1941 13 .37 59.57 .00 .00 871. 50 65.10 .00 .00 261.51 41. 93 72.94 1942 13 .37 59.57 .00 .00 644.08 72 .10 .00 .00 205.02 39.92 72.94 1943 13 .37 59.57 .00 .00 478 .14 1988.00 1260.33 .00 240.75 30.04 72 .94 1944 13 .37 59.57 .00 .00 922.16 180.10 .00 .00 281.00 24.1l 72.94 1945 13 .37 59.57 .00 .00 772 .43 1099.50 699.24 .00 285.61 66.62 72.94 1946 13 .37 59.57 .00 .00 880.76 .00 .00 .00 257.78 43.35 72.94 1947 13 .37 59.57 .00 .00 593.39 119.70 .00 .00 202.63 63.36 72.94 1948 13 .37 59.57 .00 .00 500.87 142.50 .00 .00 184.31 37.74 72.94 1949 13 .37 59.57 .00 .00 423.86 .00 .00 .00 138.95 23.72 72.94 1950 13.37 59.57 .00 .00 235.70 13 .30 .00 .00 82.52 18.96 72.94 1951 13 .37 59.57 .00 21. 56 112.50 4.50 .00 .00 39.29 6.90 51.38 1952 13 .37 59.57 .00 38.01 33.23 553.10 .00 .00 100.40 20.18 34.93 1953 13 .37 59.57 .00 .00 471.19 127.50 .00 .00 170.44 31.86 72.94 1954 13 .37 59.57 .00 .00 387.17 1858.90 1123.02 .00 228.68 76.66 72.94 1955 13 .37 59.57 .00 .00 898.08 409.70 130.62 .00 295.96 66.76 72.94 1956 13 .37 59.57 .00 .00 875.01 18.60 .00 .00 261.67 68.72 72.94 1957 13 .37 59.57 .00 .00 627.73 57.10 .00 .00 203.94 43.62 72 .94 1958 13 .37 59.57 .00 .00 451. 56 129.60 .00 .00 171.35 45.51 72.94 1959 13 .37 59.57 .00 .00 382.38 9.50 .00 .00 128.77 20.55 72.94 1960 13 .37 59.57 .00 .00 210.72 492.50 .00 .00 164.72 41.15 72.94 1961 13.37 59.57 .00 .00 506.70 .00 .00 .00 160.06 23.83 72 .94 1962 13 .37 59.57 .00 .00 297.53 20.00 .00 .00 108.23 24.87 72 .94 1963 13 .37 59.57 .00 21.56 161.22 .00 .00 .00 55.68 9.78 51.38 1964 13.37 59.57 .00 59.57 63.95 .70 .00 .00 29.57 4.22 13 .37 1965 13 .37 59.57 1.56 59.57 25.93 .00 .00 .00 10.46 1.67 11. 81 1966 13 .37 59.57 .89 38.01 5.34 1881. 40 808.24 .00 167.20 30.77 34.04 1967 13 .37 59.57 .00 .00 908.03 .00 .00 .00 261.44 33.33 72.94 1968 -13 .37 59.57 .00 .00 606.97 145.10 .00 .00 207.17 62.10 72.94 1969 13 .37 59.57 .00 .00 534.07 136.50 .00 .00 196.20 41.63 72 .94 1970 13 .37 59.57 .00 .00 443.07 313 .30 .00 .00 191.75 46.20 72.94 1971 13.37 59.57 .00 .00 537.88 555.20 .00 .00 246.07 69.19 72 .94 1972 13.37 59.57 .00 .00 843.27 .00 .00 .00 247.55 34.97 72.94 1973 13 .37 59.57 .00 .00 557.75 305.10 .00 .00 218.94 46.05 72 .94 1974 13 .37 59.57 .00 .00 617 .01 1306.00 717.02 .00 268.57 71.27 72.94 1975 13 .37 59.57 .00 .00 935.76 1689.60 1368.96 .00 305.36 57.67 72 .94 1976 13 .37 59.57 .00 .00 935.77 628.50 335.04 .00 296.50 66.41 72.94 1977 13 .37 59.57 .00 .00 926.20 1682.60 1371.89 .00 301.14 47.01 72 .94 1978 13 .37 59.57 .00 .00 909.84 .00 .00 .00 263.49 36.36 72.94 1979 13.3759.57 .00 .00 609.77 498.00 .00 .00 253.13 71.44 72 .94 1980 13 .37 59.57 .00 .00 853.13 496.90 177.09 .00 299.88 85.40 72.94 1981 13 .37 59.57 .00 .00 885.53 .00 .00 .00 258.85 38.52 72 .94 1982 13.37 59.57 .00 .00 592.25 .00 .00 .00 183.99 31.00 72.94 1983 13 .37 59.57 .00 .00 366.33 11.50 .00 .00 125.11 19.63 72.94 1984 13 .37 59.57 .00 .00 199.41 89.70 .00 .00 84.80 13.98 72.94 1985 13 .37 59.57 .00 21. 56 145.35 .00 .00 .00 49.59 8.82 51.38 1986 13 .37 59.57 .00 59.57 53.20 .00 .00 .00 24.94 5.54 13 .37 1987 13 .37 59.57 .00 48.49 20.43 158.10 .00 .00 48.22 15.11 24.45 Mean 13 .37 59.57 .22 11.75 320.79 130.49 .00 172.19 35.84 60.97 G-B-IO Table G-B.6 Output of Reservoir Simulation for Selika Dam to Determine Reliability of Yields in the Early Years Selika reservoir simulations Parameters read from file: sel.dat Selika inflows read from file F\SELFl with adjustment fac:or 1.000 Rainfalls read from file : R\M040-RAI with adjustment fac:or 1.000 Month 1 2 3 4 5 6 7 8 9 10 11 12 Primary demand factor 1.00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 1. 00 Secondary demand factor .89 1. 62 1. 95 1. 68 .83 .51 .17 .22 .59 1.42 1. 74 .37 Evaporation (mm) 179. 208. 208. 207. 172 . 163. 128. 109. 90. 81. 108. 147. Evaporacion factor = 1.000 Operating procedure applied in following months 0 N D J F M A M J J A S 0 0 0 0 0 0 0 0 0 0 0 0 Reduced supply to be met = 80.0 % of demand Selika Storage/Area/Stage curves Volume (mcm) .00 .04 5.00 42.70 238.80 773.50 1790.00 3427.00 Area (km2) .00 .05 1.90 16.30 62.30 150.60 255.00 402.20 Elevation (m) 767.00 770.00 775.00 780.00 785.00 790.00 795.00 800.00 Maximum reservoir scorage 1000.000 Minimum reservoir storage = 2.000 Simulations based on demand growth pattern for 10 years Demand first applied in month 4 held constant for each year as: Year 1 2 3 4 5 6 7 8 9 10 Primary .00 .00 3.70 5.70 7.70 9.30 10.90 12.60 14.40 16.30 Secondary .00 .00 37.20 74.50 74.50 74.50 74.50 74.50 74.50 74.50 Number of months of failure in each year Year 1 2 3 4 5 6 7 8 9 10 Dam commissioned in 1924 Primary 0 0 0 0 0 0 0 0 3 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 8 12 4 Dam commissioned in 1925 Primary 0 0 0 0 0 0 1 10 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 8 12 4 8 12 4 8 Dam commissioned in 1926 Primary 0 0 0 0 0 0 3 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 8 12 4 8 12 4 8 4 Dam commissioned in 1927 Primary 0 0 0 0 0 1 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 12 4 8 12 4 8 4 0 Dam commissioned in 1928 Primary 0 0 0 0 2 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 8 12 4 8 4 0 0 Dam commissioned in 1929 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 8 12 4 8 4 0 0 0 Dam commissioned in 1930 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 12 4 8 4 0 0 0 0 Dam commissioned in 1931 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 secondary 0 0 0 8 4 0 0 0 0 0 G-B-l1 Table G-B.6 (Continued) Dam co~~issioned in 1932 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 secondary 0 0 5 4 0 0 0 0 0 0 Dam commissioned in 1933 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1934 primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1935 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1936 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam co~~issioned in 1937 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Darn commissioned in 1938 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1939 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1940 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1941 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1942 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 8 Dam commissioned in 1943 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 8 4 Dam commissioned in 1944 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 8 4 0 Dam commissioned in 1945 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 8 4 0 0 Dam commissioned in 1946 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 5 12 12 4 0 0 0 D~~ commissioned in 1947 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 8 12 4 0 0 0 0 G-B-12 Table G-B.6 (Continued) Dam commissioned in 1948 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 12 12 4 0 0 0 0 0 Dam commissioned in 1949 Primary 0 0 4 1 0 0 0 0 0 0 Reduced 0 0 3 1 0 0 0 0 0 0 Secondary 0 0 6 4 0 0 0 0 0 0 Dam commissioned in 1950 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1951 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1952 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1953 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1954 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1955 Primary 0 0 0 0 0 0 0 0 0 5 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 8 Dam commissioned in 1956 Primary 0 0 0 0 0 0 0 0 0 6 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 4 0 0 0 0 8 12 12 Dam commissioned in 1957 Primary 0 0 0 0 0 0 0 0 3 1 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 5 12 12 4 Dam commissioned in 1958 Primary 0 0 0 0 0 0 0 12 1 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 8 12 4 0 Dam commissioned in 1959 Primary 0 0 0 0 0 2 12 1 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 8 12 4 0 0 Dam commissioned in 1960 Primary 0 0 0 0 0 1 1 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 8 12 4 0 0 0 Dam commissioned in 1961 Primary 0 0 9 12 12 0 0 0 0 0 Reduced 0 0 9 12 12 0 0 0 0 0 Secondary 0 0 10 12 12 4 0 0 0 0 Dam commissioned in 1962 Primary 0 0 4 12 0 0 0 0 0 0 Reduced 0 0 4 12 0 0 0 0 0 0 Secondary 0 0 11 12 4 0 0 0 0 0 Dam co~missioned in 1963 Primary 0 0 12 0 0 0 0 0 0 0 Reduced 0 0 12 0 0 0 0 0 0 0 Secondary 0 0 12 4 0 0 0 0 0 0 G-B-13 Table G-B.6 (Continued) Dam commissioned in 1964 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1965 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1966 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1967 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1968 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1969 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1970 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1971 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1972 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1973 Primary 0 0 0 0 0 0 0 0 0 0 (..~)~( Reduced 0 0 0 0 0 0 0 0 0 0 (l\~1t,...E:.\ Secondary 0 0 0 0 0 0 0 0 0 O· .~ R. .1":1.. Dam commissioned in 1974 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 I Cq':r Secondary 0 0 0 0 0 0 0 0 0 0 .. :: ... --~ - . ~--,~- Dam commissioned in 1975 Primary 0 0 0 0 0 0 0 0 0 0 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 0 Dam commissioned in 1976 Primary 0 0 0 0 0 0 0 0 0 3 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 0 8 Dam commissioned in 1977 Primary 0 0 0 0 0 0 0 0 1 12 Reduced 0 0 0 0 0 0 0 0 0 0 Secondary 0 0 0 0 0 0 0 0 8 12 Summary of reliability (% of months) Primary 100.0 100.0 95.5 96.1 97.8 99.4 97.4 96.5 98.8 95.8 Reduced 100.0 100.0 95.7 96.1 98.1 100.0 100.0 100.0 100.0 100.0 Secondary 100.0 100.0 84.6 81.9 84.0 87.7 88.1 87.0 88.3 88.9 Summary of reliability (% of years) Primary 100.0 100.0 92 .6 94.4 96.3 94.4 92.6 94.4 92 .6 90.7 Reduced 100.0 100.0 92 .6 94.4 98.1 100.0 100.0 100.0 100.0 100.0 Secondary 100.0 100.0 79.6 72 .2 75.9 79.6 79.6 79.6 81.5 81.5 G-B-14